My friend Amelia bought some of those 1 gallon tanks with the "undergravel" filters for her guys. Honestly, I don't see how they can be considered "filter's"... if yours is anything like hers.
All the "filter" consists of is a plastic grate (with holes in it), a tube, and an airstone. So if it is a "filter" what filters the waste? Where is the waste stored? How could that possibly keep the water clean?
The only thing I can think of..is it "filters" thru the holes and accumulates on the bottom of the tank, and the water really isn't being cleaned at all imo. And with it only being 1 gallon..it's quite difficult to use a gravel filter in that depth of water, most of the gravel vaccums work where you have to shake the vaccum end in the water to get it flowing..and those 1gal tanks are just to shallow.
I would buy one of those small sponge filter kits they sell. The air flow is adjustable and the filters are disposable...or you can wash them out in old tank water. The entire starter kit cost about $11 at Wal-mart and the refill cartridges are $4 for 2.
Her fishies HATED that tube with all those bubbles. There was no way to reduce the flow and it was so strong..it made the surface stay all bubbled up all the time. The fish looked very stressed and sickly (lost color and stay on the tank floor all the time). She put in the sponge filter and now they are all acting their usual perky selves.
Honestly, I would just get a 2.5 gal tank setup. Those come with a regular filter setup, can support a heater with no problems, and just seem a better deal then those 1 gal setups they sell, heck they're only about $8-10 more at Wal-mart.