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I have a 75 gallon freshwater aquarium with a pretty heavy fish load (7 giant fist-sized+ blood parrots). But I also have a giant Eheim 1200XLT canister filter running with gallons of various bio media, so my ammonia and nitrites consistently read 0 ppm.

I came across the equipment to essentially build a moving-bed filter.

Will the extra bio-filtration provide any additional benefit for my aquarium?

Apart from the fun of the build, I'm not sure if there is any benefit to a moving-bed filter beyond reducing the (already zero) ammonia/nitrites.

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It would provide additional capacity for biological filtration, but it wouldn't likely provide any additional benefit.

Basically, the nitrogen cycle works to convert ammonia to nitrite, to nitrate, and finally to nitrogen gas, the first 3 compounds being toxic to fish, with the level of toxicity decreasing in that order. The problem that we all experience in small fish tanks without specialized filtration, is that we cannot accomplish the final step of converting nitrate into nitrogen gas. The reason being that the final step requires bacteria in an anoxic (oxygen-free) environment.

Any additional traditional filtration, such as the product you're asking about, simply provides more surface area for more normal nitrifying bacteria to inhabit. At best it would just make your tank better at creating nitrate. But still, the bacteria in a tank will only grow in numbers to what is required to process the waste in the tank. Your existing eheim filter, as well as the surfaces of everything in the tank, likely have more than enough area for your stocking, and probably could handle many times more, specifically speaking of processing ammonia. Bacteria are just really good at what they do. The amount of surface area required for nitrification is usually only a fraction of what we actually use to filter our tanks. Unless you're experiencing ammonia or nitrite spikes on an established tank, you have enough filtration for your bio load and feeding regime.

Side note, if you are able to find a filter that can process nitrate into nitrogen gas (denitrification), it can greatly reduce the need to do water changes, or in some situations can remove the need to do water changes entirely. In saltwater tanks, there's a number of fairly simple methods to accomplish this, some of these work in freshwater as well. But, in freshwater, the cost, complexity, and simply how cheap fresh water is, makes them unnecessary for most freshwater aquariums. If you do want to research some of these, coil and sulphur denitrators, are the two most common methods that work with both fresh and saltwater. There are others such as deep sand beds and carbon dosing with varying results in freshwater, the later may not work at all since skimmers aren't used on freshwater tanks.

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