I am utterly confused by that statement. Filtration is nearly always both mechanical and biological.
I'm not confused. You're absolutely right (with an occasional exception, such as fluidized bed sand filters which don't do mechanical filtration, unless you add a pre-filter). Virtually any media will be colonized by nitrifying bacteria in time, given water flow, oxygen, and nutrients (ammonia/nitrite). Sure, the extent of mechanical filtration will vary with fluid dynamics or water flow and exactly which media.
Excess nitrate has nothing to do with nitrite levels and excess deterius in the tank wont kill beneficial bacteria, causing a mini-cycle and therefore nitrites to be present.
More or less true. Naturally, more ammonia--> nitrite = more nitrates produced. Under some conditions, de-nitification (anaerobic) can produce nitrite from nitrate. A sludge filled tank can affect redox, which won't cause a mini-cycle but can affect how efficiently wastes are broken down.
Response from Seachem:
Thanks for the email and for your interest in our opinion. It is very important to clean your mechanical filter (sponges, filter floss, wool, etc) often and properly. This will help to keep the rest of the filter media (chemical and biological) running more efficiently.
By cleaning your mechanical filter out often, you are removing any large, undissolved particulates that could potentially exhaust your chemical media quicker or clog your biological media. Neither is good.
More or less agree, and I'm one who prefers keeping sludge, mulm, whatever, low. Which means I vacuumed gravel (back when I used to have gravel) thoroughly with each water change. Sand is a different story, partly because de-nitrification can take place at varying oxygen gradients in the sand. I don't want high sludge/low oxygen conditions in my filters. But
how often to clean media varies and, quite obviously, completely depends on individual setup, variables including stock, feeding, filtration, filter flow, redox, etc.
You can have a filter combination of a fast HOB filter and a slow-ish canister flow (like classic Eheims, 2217 or smaller) where the HOB captures enough of the particulates that your canister doesn't need to be cleaned often.
You can have enough total canister filtration vs. tank/bioload and/or enough water changed, oxygenation, mineralization, and various other aspects of redox, such that waste is broken down efficiently enough to require only occasional filter cleaning, or you can have the reverse and need frequent filter cleaning.
I really don't see the point in debating canister filters. They're an option and personal preference that quite obviously can be thoroughly successful, especially fw tanks. Just as obviously, there are a variety of ways to approach filtration and everyone has their own preferences. And, no, canisters are not nitrate factories when well maintained-- one of my tanks is a 135 with two Eheim 2217s, weekly 30-35% water changes (sometimes more), no plants, currently 16 Cyphotilapia 4-9+ inches, plus a few other fish, filters get cleaned every two or three months on average and nitrates stay about 5.
As far as cleaning filter media, in my case: well water straight from the tap (no possible chlorine), temperature roughly matched to tank temperature, and it doesn't discernibly disturb bio media and doesn't affect water quality-- and I've experimented with cleaning 100% of the media for a tank and still no problem. Not to say this will work for everyone with varying setups and water sources. So not much help there. My filter cleaning schedule completely depends on tank and filters, varying between weekly and every couple of months or so. What I do know is
replacing media of whatever type means you reduced your bacteria colony by whatever % of it was residing on the media you disposed of, potentially an issue if you replace too much media.
Most anything else I can think of-- such as ammonia present in tap water, which can happen-- wouldn't necessarily explain why one tank would have nitrite issues and others wouldn't.
