Biological Filter Medium - Submerged or Emerged?

[woolfam]

As I understand things, beneficial bacteria grows on pretty much everything in your tank. The purpose of having some type of biological filter medium is to increase the surface area upon which the beneficial bacteria can grow. Increasing the surface area allows for a greater population of the BB, and thereby allows for better filtration / higher stocking levels / etc.

I have built a DIY Wet/Dry filter for my 29g community that would allow me to position the biological filter media (bath scrubbies) either above or under the water in the sump. In researching where to position the medium, I have read that the BB grows best in an aerobic environment, and therefore it would be better to leave the medium emerged (above the water level in the sump) to provide for the best environment for the BB.

On the other hand, canister filters seem to be all the rage. As I understand how a canister filter works, the biological filter medium is submerged.

I've run my filter for 8 or 9 months with the medium emerged, and it works well. I have been given a 55g (what a blessing!) and am going to build a DIY wet / dry / sump filter for it. Having the biological filter medium submerged would allow for a little more compact design than if I were to have the medium emerged.

My question is... How much benefit is there in having the medium emerged as opposed to having it submerged? Would it be worth sacrificing the compact-ness of the design to provide for the reportedly greater filtering efficiency of having the medium emerged?

 

[wesleydnunder]

I honestly don't think you'll notice a difference as long as you have good contact with your water and media. Some wet/dry designs have the compartment for the bioballs above the water level of the sump and they are good biofilters. The only drawback I see with having that arrangement is if you have a power loss for long enough that the media completely dries out, the bacteria won't survive. If the media is immersed this isn't an issue.

Also, if the water in your tank contains enough oxygen for the fish to live, it has enough to sustain aerobic bacteria.

Having increased surface area will only increase bacteria levels if there is sufficient food for the additional bacteria. If your tank surfaces and original filtration are enough to sustain bacterial levels equal to the current food source, only an additional amount of food will increase the bacteria count. Most of the time, overfiltering (the addition of several filters on one tank) only serves to have more space for the bacteria to live, but the number of bacteria simply spread out among the different areas, in stead of increasing their number. Again, bacterial colony will only grow to the size of available food source, not the size and number of "houses".

Mark