Originally Posted by lakemalawifish
Just reading all of this and realize I left out several crucial things in my attempt to help, thanks ya'll for filling in the blanks! Not sure what type of filter you plan to use on this 1 gal tank but I agree a sponge filter is an absolute and of course a heater. Do they make a HOB for a 1 gal? Just curious, never seen one. If you have a HOB for this tank you can cover the intake with either the sponge cover you can get those at pet stores, or make one yourself if you have some sponge filter material. I have even cut the netting off a fish net (the ones with the very tiny holes) and attached it to the filter intake with cotton thread. The pet stores in our area run $1 a gal tank sales, but if you have access to Craigslist you can usually find good deals there also. I agree, a 1 gal tank is going to be hard to maintain.
I wouldn't recommend placing a HOB on a 1 gallon aquarium due to their "waterfall" tactics which will highly disrupt the fish in such a small aquarium bringing un-needed stress into the environment. Fry are extremely fragile and can be gushed away in such instances.
Your recommendation of a small sponge filter geared towards that size aquarium would be the best option at this point, although ideally it would be better to only house fry in this aquarium for a small amount of time before transferring them to a larger "grow-out" environment. Also please ensure if you intend to sponge this tank, that you get an air filter (which will power the sponge filter) that is geared low enough for that sponge filter & aquarium. You will have a lot of unnecessary water movement with a high air filter on such a small aquarium.
As stated by others you should also put an aquarium heater in the tank to ensure the temperature stays at a set degree to avoid stressing the fry. Water changes in such a small environment (1 gallon) can likely lead to unwarranted stress as well due to the vast majority of water that will be taken out and then replaced. You will also need to take into account that you have to be VERY precise on water quality and water temperature when replacing the water.
Another essential piece of the puzzle for fry keeping, is security. No matter what may be in the tank. I always recommend having cover for the fry as this gives them a sense of security and removes any unneeded stress from the aquarium.
I recommend doing as lakemalawifish stated and searching craigslist or local shops for a small aquarium (10 Gallon should suffice for most fry) and having that setup as your fry factory.