Have a realistically new 50 gallon freshwater tank with 2 marineland bio wheel 330 filters, a marineland air pump and a single heater keeping the water at 75 steady. I have 1 inch of gravel on the floor with 2 fresh water plants and a few Rocks and plastic decorations for the fish to hide. I filled the tank with rocks and water and let it run for 2 days before I bought fish. I have 7 small tiger barbs and they seem happy and healthy swimming on all levels in the tank. I have been unable to get any new fish for 3 weeks because every time I test the water, my ammonia is at a 6 and I don't want to kill the new fish. How do I fix this problem, and not a temporary fix. I have gone from feeding the fish every day to feeding them every other day. I do not over feed them either, there are never leftovers. I have not used ammo-lock because it seems to be a temporary fix and I want a long term solution. Please help me!!