Made my first seachem excel purchase and I did as the bottle requested. Start with a water change then dose accordingly. I did so and in less than 24 hours I have a very cloudy tank and almost all of my fish are hanging around on top of the tank.
I've read that excel can mess with oxygen which would result in my fish hanging up top. But what's with the cloud? I'll always have a mild cloud a day after a normal water change, but I've never seen it this heavy.
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.