....For the 10 gallon tanks I have some inexpensive Wally-World air-stones that stretch nearly edge-to-edge lengthwise in the aquariums. They are placed at the very bottom of the tanks where they are nearly covered with gravel. ......
I wonder if the nearly covering the air stones in the pea gravel has set up a bio filter of sorts with bacteria collecting on the gravel....
Under gravel filters work that way although they usually have small activated charcoal pods on the out flow tubes.
Yep, that's my thought on it too. I use sponge filters a lot. Look them up. I figure if you are pumping air into the tank, you might as well yield all you can from that by adding a filter to it. About as cheap a system as you can get. I started adding a sponge to some of the regular aquarium systems (like the what Phil mentions the 350 ) so the fry don't get sucked up. I swear that doubles the bio filtration abilities. They are a lot easier to clean than a gravel floor. But yes, the gravel with air in Catmandoo setup seems like a "coarser" version of a sponge as a biofilter.
I still believe the charcoal has nothing to do with Biofiltration. I was told by a very knowledgeable aquarium guy that all it does is make you water gin clear. People don't want dirty looking water.
