Filtration for a goldfish tank

Question
Hi, Nori. I am having trouble keeping my 35-gallon goldfish tank clean. It houses one large (7-inch) shubunkin, plus four smaller goldfish and a plecostomus. I am running an Aquaclear power filter on it that is rated for 30- to 50- gallon tanks, but I feel like there is still a lot of gunk in the tank when I vacuum the gravel. (I vacuum, and change about 20 percent of the water, about once a month, in addition to adding a gallon a week for evaporation.) I also keep a live plant in the tank. I recently had an outbreak of tail rot that is proving hard to clear up, though tests for ammonia, nitrate, and nitrite have been negative. I feed once daily. Should I replace, or maybe supplement, my filter? (I need to stay with something that's not too expensive.) I'd appreciate any suggestions. Thanks.  

Answer
Hi Abbey,
Thank you for your letter. As an outside party, I would guess that your problem is over feeding and the solution is to feed only a couple flakes per square inch of fish. Also, if the plant starts to turn brown, throw it out, or throw out the brown parts. You can also use Cycle, friendly bacteria, in the tank.
Please do not use chemical treatments. You don't need a larger filter.
For further information, please read and print out this page:
http://steamboats.com/aquarium.html
Write back if you want to discuss further.
Nori