Copyright 2000 by George and Karla Booth
Plants, like all living things, need the proper nutrients to thrive. Unlike animals which can hunt for food, plants depend on the environment to supply them with the basic nutrients needed for survival and growth. The mechanism that drives plant growth is photosynthesis and photosynthesis requires light for energy and CO2 to drive the chemical reactions. Various other elements are needed during photosynthesis to create the carbohydrates which are stored and later used for growth.
The process of photosynthesis requires a specific light energy threshold. If the light is not bright enough, photosynthesis will not occur. Beyond that threshold and up to some high light level, photosynthesis will run faster and faster. Depending on aquarium depth and the types of plants, 2 to 3 watts per gallon of "full spectrum" fluorescent light is normally recommended. ("Watts per gallon" is not the most accurate way to specify the light levels needed but unfortunately it is about the only reliable measure that is available to aquarists).
It is important to note that if any one of the nutrients required for photosynthesis is in short supply, that single shortage will be the limiting factor for the photosynthesis rate. Typical aquariums are deficient in both CO2 and iron (Fe) with respect to optimum levels. Various "trace element" products are available to the hobbyist to meet iron and other trace element requirements. CO2 injection can be used to boost the concentration of CO2 in the aquarium.
In high concentrations, CO2 can block the respiration of CO2 from the fishes gills and cause oxygen starvation. Since the gills depend on a concentration differential between CO2 levels in the blood and the water to transfer gases, high CO2 levels in the water will reduce the amount of CO2 that can be transferred from the blood. Also, high levels of CO2 in the blood will cause acidosis (see "Andromeda Strain" by Michael Crichton). Although different references have wildly varying values for toxic levels, a concentration of below 30 milligrams per liter (mg/l) is definitely safe. Based on our experience, plants do best at around 15 mg/l of dissolved CO2.
It is a common misconception that water can hold only so much dissolved gas and adding CO2 will displace oxygen. This is not true. As a matter of fact, if enough CO2, light and trace elements are available to enable vigorous photosynthesis in a heavily planted aquarium, oxygen levels can reach 120% of saturation. Even at night, when the plants stop using CO2 and start using oxygen, the oxygen levels will stay about the same as a typical non-planted aquarium.
In some regions of the country, the water supply is in close contact with limestone. These regions have "hard water" which typically has high pH (greater than 7.8).
There are two things at work here. Limestone is mostly calcium carbonate (CaCO3) which slowly dissolves in water and dissociates into Ca++ and CO3-- (calcium and carbonate ions). The calcium ions (Ca++) contribute to general hardness, abbreviated GH. Magnesium ions (Mg++) are the other component of general hardness. GH does not affect pH but is the type of hardness that biologically affects organisms. Calcium is used in the walls of cells and a lack of calcium can cause both plant and animal deformities. If a plant or fish is said to prefer hard or soft water, it is "general hardness" that is being discussed.
The carbonate ions (CO3--) are a component of alkalinity, also known as acid buffering or carbonate hardness (KH). KH measures carbonate and bicarbonate ions and is the predominant component of alkalinity. In general, KH is a prime determinant of pH. High KH means high pH. The carbonate buffering system is the "natural" buffer for water.
GH and KH can be measured in "degrees", "parts per million (ppm)" or mg/l. None of these are very useful in chemistry work and are not used by professional chemists. However, most test kits use one or the other of these terms so hobbyists generally use them (the "degree" term is more common). One degree KH (dKH) or GH (dGH) is "equivalent" to the hardness generated by 17.8 mg/l of CaCO3, i.e., even if the hardness didn't come from CaCO3, it has the same effect as if CaCO3 was used. CaCO3 is the most common source of hardness, so it is the standard of measurement.
I'm glad you asked. You can reduce pH by injecting moderate levels of CO2 into the aquarium water.
CO2 has a natural equilibrium in water of about 0.5 mg/l. In an aquarium with a moderate fish load, the fish may raise the concentration to 2 to 3 mg/l. This is far below what plants need to really grow well. You would not get beneficial levels of CO2 for the plants unless you had the fish packed in gill to gill.
Likewise, decomposition of waste material in the water by bacteria and other processes releases some CO2, but again the levels are small compared to the needs of rapidly growing plants.
It depends on the amount of carbonate hardness present. KH, pH and dissolved CO2 levels have a fixed relationship as long as carbonate is the only buffer present. You cannot determine CO2 levels from pH readings alone. Also, the presence of commercial phosphate buffers like "pH-UP", "pH-DOWN" and "Discus Buffer" greatly complicates the relationship.
A dissolved carbon dioxide test kit is handy but optional. The Lamotte CO2 test kit is our favorite. It is easy to use and costs about $25. Note that the accuracy of the test kit can be affected by high levels of mineral acids in the water.
You can also determine CO2 levels from accurate measurements of KH and pH and a KH/pH/CO2 chart. Lamotte makes a great narrow range pH test (6.5 to 7.5) that you can interpolate down to 0.05 pH units. The Tetra KH test kit seems about the best for use with the table and is very inexpensive. We also have a Lamotte alkalinity kit (KH) but prefer the Tetra kit. To determine the CO2 level in your water, measure KH and pH and look up CO2 in Table 1.
The following table is from a Finnish aquaria magazine (Akvaariomaailma) and was posted by Pauli Hopea on the Internet.
----------------------------------------------------------------------- \ pH | 6.0 6.2 6.4 6.6 6.8 7.0 7.2 7.4 8.0 KH\ | ----------------------------------------------------------------------- 0.5 | 15 9.3 5.9 3.7 2.4 1.5 0.9 0.6 0.2 1.0 | 30 19 12 7 5 3 1.9 1.2 0.3 1.5 | 44 28 18 11 7 4 2.8 1.8 0.4 2.0 | 59 37 24 15 9 6 4 2.4 0.6 2.5 | 73 46 30 19 12 7 5 3 0.7 3.0 | 87 56 35 22 14 9 6 4 0.9 3.5 | 103 65 41 26 16 10 7 4 1.0 4.0 | 118 75 47 30 19 12 6 5 1.2 5.0 | 147 93 59 37 23 15 9 6 1.5 6.0 | 177 112 71 45 28 18 11 7 1.8 8.0 | 240 149 94 59 37 24 15 9 2.4 10 | 300 186 118 74 47 30 19 12 3 15 | 440 280 176 111 70 44 28 18 4 ----------------------------------------------------------------------- | CO2 milligrams/liter -----------------------------------------------------------------------
An optimum level of CO2 is 15 mg/l; a good range is 10-20 mg/l. Keep in mind that good CO2 levels also require good light levels, proper nutrients and proper trace elements to be effective in growing plants. All things must be in proportion.
The table is very accurate but your measurements may not be! There are two concerns.
First, you need to think about the resolution and accuracy of the test kits. If the KH kit is accurate to +/- 0.5 dKH and the pH kit is +/- 0.2 units, the range of CO2 values read from the table can be quite large. Let's say you get a carbonate hardness reading of 3 dKH (+/- 0.5) and get a pH reading of 7.0 (+/- 0.2). This would indicate CO2 ranging from a low value to an optimum value:
KH = 2.5, pH = 7.2 >> CO2 = 5 mg/l KH = 3.0 pH = 7.0 >> CO2 = 9 mg/l KH = 3.5 pH = 6.8 >> CO2 = 16 mg/l
Also, most KH test kits actually measure total alkalinity. If KH is the predominate component of alkalinity, all is well. However, if other buffers are in the water (phosphate buffering products, for example), you will get a higher "KH" reading and you will think you have more CO2 than you really have.
In most cases, the aquarist balances KH (which raises pH) with CO2 (which lowers pH). For example, we are blessed with soft tap water, both in terms of KH and GH. We add sodium bicarbonate (NaHCO3, baking soda) to get 4 degrees of carbonate hardness (4 dKH) as measured with a Tetra KH test kit (inexpensive but effective). With a typical equilibrium dissolved CO2 level of 2-3 mg/l, this gives us a "natural" pH of around 7.7. We then inject CO2 to lower the pH to 6.9, giving us a dissolved CO2 level of 15 mg/l, which is just about perfect.
That depends on the pH you would like and how much CO2 you want to inject. The pH should be set to suit the species of fish you will keep in the tank. For a community tank, a neutral pH (7.0) is a good target. As mentioned before, a good level of CO2 is 15 mg/l. To figure out the proper KH for this community tank, use Figure 2. Find the 15 mg/l CO2 line and go across the chart until you get to the pH=7.0 line. Go down from there to determine the amount of KH you should have (5 dKH). Adjust your water to have 5 dKH and then inject CO2 until the pH is 7.0. That's all there is to it!
If you need to add KH, add small amounts of sodium bicarbonate (baking soda, NaHCO3). One teaspoon of baking soda will increase the hardness of 50 liters (13 gallons) of water by about 4 dKH. If you want to raise both KH and GH, two teaspoons of calcium carbonate (CaCO3) will raise the hardness of 50 liters of water by 4 dKH and 4 dGH. It is best to adjust the hardness slowly, using a test kit to measure the changes as you go along.
If your tap water already has a KH higher than you desire, you have two choices. The first choice is to use more CO2 to achieve the pH you want (but keep CO2 less than 30 mg/l, of course). Another choice is to use reverse osmosis (RO) to lower the KH of your tap water. Pure RO water has no KH so you can mix RO water and tap water to get the KH you desire.
One thing to keep in mind: for good fish health, it is better to have a stable pH than a theorectical optimum pH. Most fish books give the desired pH as the pH that is found in the natural environment of the fish but farm raised fish will most likely come from water of neutral pH (7.0). If the KH of your tap water and good levels of injected CO2 produce a pH anywhere in the 6.5 to 7.5 range, it is best to not attempt to alter the KH of your tap water. The fish will adapt just fine to the resultant pH and they will appreciate the stability (and lack of extraneous chemicals!).
As far as we are concerned, none of the systems offered by the aquarium industry are cost effective. Various bits and pieces are good, but no one system combines high quality, high capacity and low price. If you have access to the right suppliers, you can assemble a much more capable system for less money. Here is what we recommend:
Get a 5 pound to 20 pound CO2 tank from a welding supply shop. Welders use CO2 to provide an inert atmosphere so they should be easy to find. If not, check for compressed gas dealers. Other sources include fire extinguisher suppliers and beverage dealers. Careful shopping may uncover a used tank that will be more cost effective. We pay $70 for a new 5 pound tank and $120 for a new 20 pound tank.
The place where you buy the tank should be able to refill it. Some will refill while you wait, some will trade a full bottle for an empty, some will send it somewhere to get filled. Naturally, you don't want it disconnected from the aquarium for very long or your pH will shoot up. We have an old 2.5 pound bottle we connect up while we take the regular bottle to be filled.
In our area, a refill for a tank 20 pounds or smaller is the same price ($10), so the bigger tank saves money in the long run. A 10 pound lasts more than twice as long as a 5 pound, and a 20 pound lasts more than twice as long as a 10 pound (they have trouble getting all the pounds into the tank, so 0.5 pound out of 5 is a higher percentage than out of 10, etc).
The best is a single stage, two-gauge regulator designed for use on welding gas cylinders. This reduces the 950 pounds per square inch (psi) tank pressure to 10-20 psi. You can get a cheap fixed-pressure regulator with no gauges for about $45 from a home brewing store. The lowest cost regulator is a Flow Regulated Orifice Gauge (FROG) which puts out a constant pressure (22 psi or so). These are cheap but there are no gauges so you don't know when the bottle is about empty. Without gauges, you'll just be surprised some day when you check the controller and find your pH is 8.2.
We paid $70 for an adjustable regulator with high and low pressure gauges. The high pressure gauge tells you when the bottle is about empty. The bottle pressure will stay at 950 psi as long as there is liquid CO2 in the bottle. Once the liquid is gone, the pressure will begin to drop (2-4 weeks to go from 950 psi to 200 psi). When the pressure is around 200 psi, you should recharge the bottle since the regulator gets a little unstable at that point.
Another option is called a Regulated Flow Meter which is essentially a regulator with an integrated needle valve control plus a flow rate gauge. Although the flow rate gauge is not that useful for our purpose, the integrated flow control is very smooth and precise. There is also a dial gauge that gives you the pressure inside the tank. This is by the Victor Equipment Company of Denton Texas, model # HRF1425-580 and costs about $65.
For an automated setup with a controller, a solenoid is needed to shut of the CO2 flow. We found a commercial unit for about $60. It's definitely over-kill but it has never broken. It's good for 200 psi and it's non-corrosive. If you have a local commercial plumbing supply store, they might have them. Also look up "valves" in the phone book.
The solenoid goes after the regulator on a CO2 cylinder, i.e., the low pressure side. Some solenoids use high inlet pressure to help keep them turned off and may not work in this application.
The regulator usually has a 3/8" NPT thread and the solenoid may have a 1/4" or 1/8" NPT thread, so you will need to find a place that sells up and down thread adapters. Most good hardware stores and plumbing supply places have these.
After the regulator, you will need a fine control valve to get the extremely slow flow rate you need (1-3 bubbles per second for a manual setup). Typical aquarium needle valves won't work - you will find they are either all on or all off in this application. A decent valve will run from $10 to $40 depending on what you find. Check the Yellow Pages under "valves".
We have used one by NuPro (model B-4MG2, $35) that works extremely well. The ARO Corporation, phone (419) 636-4242, makes two models, the F01 and F02, which sell for less than $15. Another valve is the Parker N400B-11AC. One more is by Whitey, Model no. B-ORF2.
To help set the flow you would need a "bubble counter" or some way to see the actual flow. If you inject directly into the tank or you can see how much is going into the reactor, this is optional. We like the Dupla bubble counter (about $40) because it also includes a check valve. You don't want water to get into the regulator (water + CO2 = carbonic acid, which is mildly corrosive)! Aquarium air line check valves may work, but the one I once used had an internal melt down when presented with carbonic acid.
The CO2 regulator should be set to about 20 psi on the output. Typical regulators get a little unstable at pressures below that. The fine control valve should be adjusted to give you about one to three 1/8" bubbles per second (the rate that is best for your specific tank will vary depending on all kinds of factors).
Most aquarists start with a "manual" system where a small but constant flow of CO2 is used. Advanced aquarists use automatic controllers that measure pH and adjust the CO2 accordingly.
A manual setup works fine but takes constant fiddling to balance it. The CO2 bubbles into the water continuously and you have to balance the inflow against plant usage and diffusion into the air. Having a quality fine control valve is very important in a manual setup. Because the plants will use CO2 when the lights are on and produce CO2 when the lights are out, you will get minor pH fluctuations of about 0.3 over the course of 24 hours.
Don't shut the CO2 off at night in a manual system. Many people assume you have to turn off the CO2 at night because the plants won't be using any and too much will build up in the aquarium. This is not true. Much more CO2 is lost to atmospheric diffusion than is used by plants. If you turn the CO2 off at night, the water will return to equilibrium levels by morning. This has two consequences: it will take longer to get the CO2 level back to optimum and the pH swings between day and night will be much greater, stressing your fish.
In our 85 gallon tank without a controller, we have a constant slow flow of CO2 that just balances the diffusion rate and plant usage and maintains 7.0 +/- 0.2 (higher in the daytime when plants are using CO2, lower at night when plants and fish respire CO2). These pH swings have caused no problems with fish ranging from discus to angels to rainbowfish to small tetras.
An automatic system is great if you can afford it. A controller costs around $250-300 and the solenoid is another $60. Electrodes must be cleaned every few months and replaced every 18 months or so ($40). But a controller will keep the pH very stable.
We use electronic controllers to maintain a stable pH on most of our tanks. The controller measures the pH with a special electrode. If the pH is above the desired "set point", the controller turns on the CO2 to lower the pH. When the pH gets below the set point, the CO2 is turned off. Since CO2 will try to equilibrate with the atmosphere, it will slowly diffuse out of the water (and be used by plants), again raising the pH. So the pH will slowly go back and forth from 6.85 to 6.95. In another tank with a cheaper controller, it goes from 6.80 to 7.00.
Note that the automatic controllers are not perfect in that the probe can get out of calibration and return incorrect values to the controller. We check pH monthly with a good test kit to check the controller and probe calibration.
There are all kinds of ways to get the CO2 into the water. Some are very efficient (and may need a controller to avoid large fluctuations) and some are less efficient, wasting CO2 but providing a margin of safety. The basic idea is to get the CO2 bubbles in the water and keep them in contact with the water as long as possible.
The simplest method is to bubble CO2 from an airstone on the bottom of the aquarium and have the CO2 bubbles come up below a powerhead or filter outlet so they get pushed around the aquarium. Get a good airstone so the bubbles are as fine as possible.
The CO2 can also be directed into the intake of a canister filter, giving it plenty of time to mix before it comes out the outlet. One of our tanks has the CO2 bubbling into the intake of an Eheim canister filter. This works fine but is not as efficient as a purpose-built CO2 reactor.
A CO2 reactor is nothing more than a device to mix CO2 and water. We use Dupla reactors in our trickle filter sumps but almost anything that will allow the CO2 and water to mix will work. The idea is to work out a "counter current" situation where water flows from top to bottom in a large tube and CO2 bubbles up against the water flow. You may be able to adapt a cheap protein skimmer for this purpose.
A very nice reactor can be made from a spare powerhead and parts you may have in your junk drawer. This uses the "counter current" principle. The gravel tube from a Python water changer is connected to the outlet of a powerhead with a short piece of 5/8" hose. To keep fish out of the powerhead, a Magnum inlet strainer is used on the powerhead inlet. At the bottom of the gravel tube, an airstone is connected to the CO2 airline. In operation, the powerhead moves water down through the gravel tube. The CO2 bubbles coming from the airstone rise against this current and collect at the top of the tube. When the system is balanced, there is about 1/2" of free CO2 at the top of the tube. Of course, this whole assembly is in the aquarium. The CO2 flow is easy to adjust since you can see the bubbles coming from the airstone. See Figure 1.
---------- | small |------------------ |powerhead| = = = = = | <- Magnum strainer | |------------------ --------- | | | | ---- ---- | | <- free CO2 |............| | o | | o | | o | <- Python gravel tube | o | | o o | | o | | o_ o | | | |o | | | | | CO2 in | | | | I | I | I I I \______________/
The injector is relatively efficient. Not much CO2 gets away in the form of free bubbles and a good level of CO2 can be maintained in the aquarium.
Certainly! What you don't want to use, however, are airstones to drive the UGF -- powerheads are the preferred method. The turbulence caused by the bubbles on the surface will greatly increase the rate at which CO2 diffuses into the atmosphere. In general, an aquarium with CO2 injection should not have a lot of surface turbulence.
Likewise, if your canister filter has a "spray bar", move it under water and point it towards the bottom to avoid a lot of turbulence. The "water fall" return from various power filters will not cause much loss of CO2.
Hogwash! Trickle filters work just fine with CO2 injection. The only change you may need to make is to not inject air into the media chamber. The slight pressure caused by an air pump will tend to push CO2 out of the chamber and cause some loss. The bacteria will get plenty of oxygen from the air being pulled down the inlet tube (that's the rushing noise you hear).
As a matter of fact, a trickle filter is the perfect place to put your CO2 reactor!
Yes, you can generate CO2 cheaply by using a yeast fermentation method. Thus will give you a relatively constant source of CO2 for a period of about 3-4 weeks that will be sufficient for smaller tanks (less than 50 gallons). If your plants perk up after a few days, you will then know that CO2 was your limiting factor.
To make the yeast fermenter, start with a 2 liter plastic soda bottle. Poke a hole in the cap so that a piece of rigid plastic aquarium airline will just fit through. Glue a short length of the airline into the hole using silicon sealer. Cut the end of the airline that goes in the bottle at a 45 degree angle to allow any moisture to drip off. See Figure 2.
Add 2 cups of sugar, 1 tsp of bakers yeast and 1 tsp of baking soda to the bottle then fill it to within 3" of the top with luke warm water (80 degrees F). Cap it with a spare cap (without a hole!) and shake it until the sugar dissolves.
Put the cap with the airline on the bottle and use flexible airline to connect the fermenter to a reactor as mentioned above. There may not be enough pressure to use an airstone -- if so, just bubble it directly into whatever you're using. However, enough pressure can build up in the bottle to burst it or pop the cap off and spray a sugary mess around your room, so don't block the outlet of the bottle! It is best to place the fermenter above water level so that water will not siphon back into the bottle if the fermentation should stop for some reason.
More information about this method is described in Karen Randall's article in the May 1997 Aquarium Fish Magazine.
[Thanks to David Webb (firstname.lastname@example.org) for this information about yeast fermenters]
There are two concerns with CO2. One is that high concentrations in the air can cause asphyxiation and can kill you. This could happen if all the CO2 in the bottle suddenly escaped in an enclosed space and you were trapped in there. How could this happen?
The CO2 in the bottle is in liquid form because it is under very high pressure (around 1000 pounds per square inch, depending on temperature). The bottle itself has a safety valve that will open if the pressure in the bottle gets too high. This can happen if the bottle gets hot or if it is overfilled.
Overfilling is a serious problem. When the CO2 goes into the bottle, it is in a very cold liquid state and makes the bottle very cold. The bottles are designed such that there is space at the top when they are filled to their proper weight. This space will be filled with pressurized gas when the bottle is at room temperature. If too much liquid is put in the bottle, this space will be too small and too much pressure will develop when the bottle warms up, causing the safety valve to release. When the valve opens, all the CO2 will escape very rapidly.
This can have two effects. If it happens in your car on the way home, your car will suddenly be filled with a CO2 cloud, your windows will frost up and you will be startled by the loud noise. This is clearly dangerous if the car is moving. You can also be asphyxiated if you don't stop and get out quickly. Because of this, you should always transport the filled CO2 bottle in your trunk or in the back of a pickup truck.
The other concern is also related to the sudden release of the high pressure in the bottle. If the bottle is unsecured and the safety valve releases or if the bottle falls over and the valve is broken off, the bottle can become an unguided missile! There have been reports of a broken bottle flying through a house and punching through walls. For this reason, it is a good idea to secure the bottle with straps when it is in use. I use a Velcro strap fixed to the aquarium cabinet.
It should be obvious from the above that the CO2 bottle should always be upright when being transported and when in use. The regulator is designed to deal with a gas and will not work if the liquid CO2 gets into it.