filter for very small tank?

i have a small tank that i use to keep frys from mommy cichlids. about 30 of them. the small tank i bought came with a simple under gravel filter. problem is, the water gets dirty quite easily and i dont want to keep cleaning everyday..

what's the smallest filter that people will recommend? the holding tank is about 50cmx30cmx15cm (LxWxH). thanks.