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Plant Filtration?

Back in 2008-2010 I've had the honour to be in business and personal relations with Charles Clapsaddle, the owner and founder of Goliad Farms, a company that for decades is a good supplier of ornamental fish for pet stores and tropical fish hobbyists. Charles is knowledgeable professional and a great person. My wife and I visited Goliad Farms and saw his hatchery. I was impressed with how well it is thought-out.

I discussed plant filtration subject with Charles many times; I even took some of his plants he uses for plant filtration: Hornwort, Bacopa and Najas grass to try this way of filtration in my home aquariums. My intention was to reduce or eliminate the need of 50% water changes a day. At the time I was in discus hobby, and this is well known that a discus setup requires massive daily water changes. Considering that the water has to be heated up to 84 degrees and I used at least 500 gallons a day, this was quite costly.

Charles's article "Plant Filtration - No Water Changes" was published in April's 2010 Tropical Fish Hobbyist Magazine, and I was still disagree with Charles's results. So, I performed my own series of tests in regular aquariums that are more common tanks in the hobby.

Links to the article are located at the end of this page. 

I set up three 30-gallon aquariums with aged tap water, no sponge filters, just airstones, and no gravel. First aquarium didn’t have plants at all, second one was loaded with a dense ball of Hornwort, third one—with Najas grass; both aquariums with plants were loaded with plants that filled about 50% of the tank volume. I let the tanks to cycle for a week and then placed quarter size golden marble angelfish, 30 fish in each tank, i.e. 1 fish per 1 gallon. I performed nitrate tests every day to see how effective plants consume nitrates. I made sure there are optimal conditions for the plants: decent light source and no excessive aeration that removes CO2 from the water. Good indication that the plants are thriving (and so, actively consume nitrates) was their fast growing rate. Initially nitrate and nitrite readings were close to zero. I found that there is no big difference in nitrate readings between aquarium with Hornwort and Najas. The results were disappointing! In a week nitrate level in the tank with no plants was 25 ppm, in both aquariums with plants – between 15 and 20 ppm. Next week readings went up to about 55 and 40 ppm, respectively. I loaded a few aquariums with plants and still use them; they improve water quality, but do not eliminate need of water changes.

I do not call in question Charles’s results, but I asked myself a question: why our results are so different? I analyzed Charles’s facility and found one big difference between our setups. I believe that all the concrete floor and sumps in his greenhouses work as a huge biological filter, similar to live rocks. I believe that the nitrifying bacteria as well as green algae that grow on all surfaces consume major part of nitrate produced by fish, and higher plants just finish the job. In my tests I deliberately did not use any additional biofiltration: no sponge filters, no gravel, cleaned walls, and all this let me make such conclusion.

To check my supposition I performed another test, similar to aforesaid one. I replaced plants with two #5-size sponge filters per 30-gallon aquarium. I used well cycled sponges from other aquariums. I found that nitrifying bacteria did the job more effectively than plants. In two weeks the nitrate level in aquarium with sponge filters was between 25 and 30 ppm, whereas in the aquarium with airstone only it raised over 60 ppm. Keep in mind that angels in the aquariums were about dollar size at the time and produced about twice as much wastes than quarter-sized ones.

I discussed my results with Charles after his article was published. His weighty argument is that the water in the greenhouses is crystal clear at full day light, so it is obvious that green algae (especially single-celled ones) do not have enough nutrients to grow, whereas with no plants water turns green. Let’s say that 80% of nitrates are eliminated with nitrifying bacteria and only 20%—by plants. In this case we’ll see the same picture: the amount of nitrates left in water (and so, not consumed by plants) will be enough for green single-celled algae to grow.

I believe that Charles’s experiments with plants are interesting and really valuable for hatcheries. Unfortunately, average fish hobbyists can’t use these means of filtration. It should be taken into consideration that in my tests at least 50% of the volume in tanks was filled with dense ball of plants. The plant mass was so dense that the fish couldn’t swim through it. The fish in such tanks are not easy to observe or to net. Other plants that Charles uses in his hatchery are water Hyacinth, emersed Bacopa or duckweed; they create shadow and prevent rooted plants to grow well. So I believe that water changes supplemented with traditional sponge or canister filters are still the only practical means to reduce nitrate level in hobbyists’ aquariums.

Charles Clapsaddle “Plant Filtration – No Water Changes!”, April 2010 TFH magazine.