that was probably my webpage you saw the $2 corner filter for the breeding tank. Ya know, I did have a fry or two get caught in the box, and had to release them, all was well after releasing them. It seemed to happen a few weeks after birth, when they get more curious? I didn't use carbon, I used porous biomedia taken from my canister filter, sadwiched between two layers of filter floss, also taken in part from the canister. Never thought of taking the top off once the filter floss assumed its shape... a great idea! You really only need the top on till the top layer of floss "learns" to stay in place.
An equally viable method would be an airstone driven sponge filter. When not in use, the sponge can go over your main tanks intake or run on the airstone in the corner of your main tank, so that it is colonized when the breeding tank is put into service.
Or keep a sponge over the intake of your main tank's filter, then put it on your breeding tanks HOB
intake when needed. And you could also seed the HOB
with colonized biomedia, and the combination of the colonized sponge and colonized biomedia would most assuredly keep aammonia and nitrite in check.
I forget where I got them, but I think BrianNY posted a link that had the intake sponges. Each sponge came with adapters so that it could be made to fit just about any filter or situation.
Yes, the sponge filters seem to have replaced the corner filters, even at the LFS
's. But, the corner filters sure are cheap, and they do work. You actually can get some amazing flow through them with a good airstone, but perhaps you might want to dial it down a bit for fry. And you can have a very neat and clean main tank (because you don't have sponges in it) yet get a fully colonized corner filter up and running in minutes from stuff in your established canister filter. And you can run a corner filter with almost any level of water that is over its top, in case lowering the water level is needed to induce spawning. Hey, its low tech and nostalgic, what's not to like?