You do know we WANT that bacteria in the tanks right? That is the same bacteria that is in the rock that is part of the bacteria cycle breaking down ammonia.
Considering I have been keeping aquarium for 20+ years I probably know lol. But it is not as simple as "we want bacteria and we want as much of it". There needs to be a balance between aerobic and anaerobic bacteria. Otherwise the nitrogen cycle will have a solid dead end at nitrate.
Canister filters generate very large flow over a media that is designed to be populated by very large number of bacteria. For the filter OP listed, we are talking about 900GPH over 1.5G of media volume. So you end up with a bio filter made up entirely of aerobic bacteria. There is no way anaerobic bacteria can live in an environment that has that much of oxygenated water turnover. On top of that, with that much of flow, everything is sucked into the canister and basically "rot" into more nitrate. That is why they need to be cleaned very often for SW tanks and they are not designed to be cleaned very often, so it is a pain to clean a large canister filter every week or so.
These are not problems for FW tanks that canister filters are designed for. Nitrate is not a big issue for FW fish to start with. Most FW fish are very tolerant of nitrate (talking about several thousands of PPM of nitrate) and even if you want to reduce it, you can throw in some plants that would suck it up like a sponge or you can make daily 50% water changes that would take less than 10mins. For my FW tank, I drain it to the garden and fill it up with the garden hose. So draining 100 gallons of water and filling it up takes about 10-15mins
. To change that much of saltwater, I would probably need to start preparations 2 days ahead.
This is the reason why canister filters were abandoned in the early days SW fish keeping. They were replaced mainly LR filtration and designs like Berlin sumps or ,in large systems pressurized, sand bed filters. All these designs have aerobic and aerobic regions, so you can have both type of bacteria.
