It may also be tiny microbubbles coming in through the return piping. If it is just "stuff", then it may be time to find a place along the way for some carbon, filter floss, or a filter sock. However since your tank is a FOWLR I'll take the hit for saying you could add a nice canister filter, like an Eheim 2217, to your system as it will trap particulates until you clean it. As a bonus, since you run a wet dry, you can clean it a lot more than normal as it won't be your only biological source.