Originally Posted by Seattle_Aquarist
Assuming you dosed per the instructions it sounds like you may have had a substantial amount of algae in the tank. Excel, along with providing carbon molecules, has algacide properties. It is likely that a lot of algae is dying off and decomposing making the water cloudy and using up oxygen. Increase your filtration, add an airstone to increase circulation, and watch your fish. With good filtration the water should clear in a day or two. The good news, you have probably killed off a lot of algae.
I have a penguin powerhead 300 that I don't use. Would that help. I'm currently running two HOB fluval 30's.