I have a penguin 350 and a cascade 300 on my 35 gallon, and a wisper 5-15 on my 10 gallon, both have UGF's also, but the 35 has a penguin 550 power head not a pump.
You need to get a bigger tank, get another tank or lose some fish. No amount of extra filtration will correct the problem at hand. Your tank is over stocked, plain and simple.Your fish dont even have enough room to swim around and be comfortable if it's over stocked, this in itself will make them more prone to diseases. If it were me I would get a bigger tank with the proper amount of filtration.
Im using an aquaclear mini on my 5 gallon allglass bow front aquarium. I like these filters because you can rinse the sponge time and time again. I dont have to keep buying cartridges.
Im with getting a new tank considering the exspense of another filter, not to mention the extra work involved. Take people's advice and get a larger tank.
That isn't overstocked, yet.. Soon it will be though, cuz those suckers grow fast and get big. Each one needs a 30g tank alone, minimum. For the sort term, I'd go with an Emperor 280. Long term, get a 55g tank for them.