Hi all,
So for this 2 inches I think its better to have a finer(middle ground) sponge so that it doesnt let things go through it too easily. Any thoughts?
I'm with "Frenchi". I like a coarser sponge and filter media that doesn't inhibit flow too much. Even with smaller internal filters I wouldn't go finer than PPI25.
If you start looking at the actual volume of bacteria you need it is relatively small, and usually what limits nitrification isn't pore space, but oxygen. This is why waste water scientists are interested in Biochemical Oxygen Demand (BOD) as the measure of pollution, rather than just the volume of the filtration system.
My suspicion would be that in nearly all filters not all the potential sites for nitrification are occupied, mainly because the water is de-oxygenated before it reaches them.
Some people have even suggested that this is an advantage because anaerobic de-nitrication of NO3 can occur in the deeper pore spaces of sintered glass or fine sponge, but lets just say I'm deeply dubious whether this is a good idea in most filtration systems.
An exception I would make would be for wet and dry trickle filters and HMFs, this is because they have large gas exchange surfaces (in contact with atmospheric oxygen in the case of the wet and dry, and atmospheric oxygen at the top and oxygenated tank water along the sides in the case of the HMF). A large gas exchange surface allows CO2 to out-gas and O2 to diffuse in along their respective diffusion gradients.
In the case of an enclosed filter medium (like a canister filter) the oxygen carried into the filter can't be replenished from the water column while it is inside the filter. If flow through the filter is slowed by fine media, or a lot of organic debris from using your filter as a syphon, then the water will become de-oxygenated and the volume of the filter material becomes irrelevant. In this scenario some denitrification of NO3 may occur, but the real problem is that an increase in the bioload, or further slow in flow speed, may lead to ammonia passing through the filter and then accumulating in the tank water with disastrous results.
If you have plants and a substrate you have "
belts and braces", but if you are entirely dependent upon the filter you are into a positive feedback loop of:
lower oxygen levels > increased ammonia > fish death > lower oxygen levels > increased ammonia > fish death etc.
cheers Darrel