Aha, I think we found your problem. You have your skimmer in the wrong section. It should never be in the last section of your sump/wet dry filter. The reason being is that a skimmer craetes bubbles which will get sucked into your return and then pumped into your tank. If you have live rock in your tank, then you can take the bio balls out of your filter and place your skimmer in that section. If you want to keep your bio balls, then you will have to add a baffle in the last section of your wet dry box with a sponge betwen the section which the water will have to go through. The sponge will stop bubbles from entering into the last chamber.