I have a Fluval 4 plus and a 3 plus filter in a 120 gallon tank for my turtles (I only fill it with about 60-70 gallons) and when I changed the filter the sponge was only 1/3 used and dirty, the rest of the filter was still clean. The red button was up and showing that it was needing to be cleaned, also my aquarium was not staying as clean as usual. Any ideas about whats going on with my filter?