Wuts up, I'm new to this forum, and new to fish keeping as well. Always wanted an aquarium and just started a 10 gallon about 3 months ago. I heard after the fact that smaller aquariums are harder to keep up than larger ones. My problem is this: I can't seem to get a bio filter going... here is the situation: I started the aquarium with 3 black skirt tetras (sorry I don't know the scientific names). After 4 weeks the ammonia level was 0ppm so I figured I was good to add more fish. I added another black skirt and 3 zebra danios. Changed 25% of the water every three weeks but ammonia level kept building... nitrite and nitrate stayed at 0. Figured maybe my filter wasnt big enough for that many fish so I bought a bigger filter. It's been 4 weeks and ammonia is still at 4ppm and still no nitrite or nitrate. The fish seem healthy... any ideas on why bio filter won't establish?