Personally the thought of using submerged media for biological filtration doesn't make a whole lot of sense to me. Give me a wet/dry with bioballs or if I'm running a canister a set of Marineland BioWheels. Biological filtration as it is being discussed here is series of oxidative chemical reactions. Submerged media are limited to the oxygen dissolved in the water. Whatever media you use the more oxygen it can get the better.
That's interesting. I'd love to know if there've been any scientific, or at least accurate and objective, tests of bio media submerged or wet/dry for comparison. It makes sense to me that given the same useful surface area, media in a wet/dry filter would process more ammonia into nitrate but we don't know the maximum capacity of the bacteria within a given surface area to do this processing. Can it approach that maximum capacity submerged in well oxygenated water? If it can already, is there room to improve wet/dry? How much more oxygen is available in a wet/dry scenario or can that be measured?
This is not to argue; I'm pretty sure the wet/dry approach is more efficient in terms of (surface area / nitrification capacity). Just, I'd love to see more publications like Diana Walstad's lovely book "Ecology of the Planted Tank" to learn more. I would love to do it myself, but I can only speak pigeon sciencese, not actually do good science.
My preference is to use media with gobs of useful surface area with adequate flow to keep all the surfaces oxygenated. That way a small total volume can house enough bacteria to do the job. Also, I use a large-grained sand for substrate so there is all that surface area in my tank (50 x 25 cm of the top 1cm of sand equals 25-30 grams of decently oxygenated sand gives about 500 or more square meters surface area depending on the texture and porosity of the individual grains - which can increase the area by many many times). I don't use aeration in my tank and the fish and snails and shrimps and other critters seem pretty happy so I have to assume there's plenty 'nuff oxygen.
I like my tank quiet, no humming or splishy-splashy sound, because it's next to the head of my bed. Maybe I can figure out a really quiet wet-dry filter. I had to dismantle the one in the hood of my tank because it was a)noisy and b) had an ugly powerhead dangling down. Oh, and because it is way too powerful for a nine gallon planted tank.
