Last update: October 17, 1998

The debate rages over the "best" type of filter for a planted tank. It is true that a heavily planted tank with a light fish load does not need much in the way of filtration. However, since the aquatic interests of Karla and myself are focused primarily on fish and we have a much higher fish population than is typical, an appropriate filter is a serious concern. We support the use of trickle or wet/dry filters, even though they are among the most expensive options.

We agree with traditional aquatic gardeners that an aquatic garden populated by a few small fish does not require any external biological filtration. Ammonium uptake by the plants and biological filtering provided by the nitrifying bacteria covering every surface of the tank and gravel will undoubtedly protect against any harmful nitrogenous wastes.

However, our large display tanks are respectively set up for full grown discus, full grown Melanotaenia spp. and, in the past, raising angelfish. Including accent fish, algae eaters and catfish, we don't think anyone would consider these tanks "lightly loaded". Since we also feed heavily to keep the fish in prime condition, we are of the opinion that external biological filtration is required in our systems. Clearly, the lush plant growth is not consuming all the ammonium, since nitrates accumulate at the rate of 7-10 mg/l per week, requiring the use of denitrators and regular partial water changes to keep the average nitrate concentration less than 10 mg/l.

Two of the tanks use trickle filters for extra biological filtration and one uses an undergravel filter. Some aquarists conjecture that "biological filtration may have a negative impact on plants". We can only say "We doubt it". If our plants are stunted by the filtration, presumably through a lack of ammonium due to rapid nitrification, we would truly hate to see how they would grow otherwise -- we currently have to trim the faster growing plants at least every two weeks.

Important indirect effects of what is typically thought of as biological filtration are the issues of mechanical filtration or water circulation. Our belief is that these two functions are important for plants no matter what the fish load and must be provided in some form, whether by a corner filter, power filter, canister filter, power heads or whatever.

Given that heavily loaded plant tanks need extra biological filtration, and water movement and mechanical filtration should be provided, the question remains whether a trickle filter is worth the expense. Beyond the added biological filtration, we believe they are valuable for the following reasons:

  1. They provide good mechanical filtration via the prefilter in the skimmer box and filter pad over the drip tray.
  2. The mechanical filter media is much easier to keep clean than in a canister filter, promoting weekly cleaning and preventing wastes from accumulating and generating excess nitrates.
  3. The skimmer removes the surface scum that can form from proteins.
  4. They provide excellent water circulation.
  5. The biomedia stays clean since the water is prefiltered before reaching it.
  6. The media chamber helps maintain high oxygen levels at night when the plants are not photosynthesizing.
  7. When properly set up, they do not remove much CO2.
  8. The sump provides a convenient place for a CO2 reactor and other equipment.
It should be pointed out that advanced reef keepers are currently moving away from the concept of trickle filters, since they feel that they are "nitrate generators". By using large protein skimmers to remove much of the waste before it has a chance to break down and allowing the live rock to perform simultaneous nitrification and denitrification, they are keeping nitrates at negligible levels. However, the aquarists that originally used trickle filters have simply removed the biomedia and continue to use the filter for the reasons mentioned above.

For aquatic gardeners that are looking for extra biological filtration due to heavier than normal fish loads or merely for added capacity as a safety precaution, we are convinced that trickle filters are an excellent choice. Smaller trickle filters without all the superfluous bell and whistles, suitable for freshwater tanks up to 100 gallons, can be had for about the same amount of money as a quality canister filter and provide far more flexibility and functionality.