not enough filter. i had a fluvel 204 which i think was for a 40 gallon tank, and i had a fluvel 304 on the tank which i think is for 70 gallons. i think i was ok, just having one fish but i wanted to have the right about of filtration on it to be safe. so i have the xp3 running, and i left the 204 on to keep the bacteria in the tank while the xp3 cycles. also took some of the bio matter out of the 304 and put it into the xp3. im probably gonna run them both until i need to use the 204 for something.