I have 2 tanks with the prefilter on and they get a little dirty, I take them off and rinse/wringe out weekly. I have 2 other tanks without and I don't see any difference.
The difference is with a filter,
1} No pods or mysids get through to your DT
2} Your fitler pad is acting as a small filter, possibly creating a small amount of nitrates from the bacteria colonizing the foam.
