[Invigor]

I would simply gravel vac and do a few water changes...much cheaper than a SUPER WHAMMO SKIMMERZ0R.

try something silly like a 50% water change 3 days in a row, gravel vaccing each time.

I wouldn't consider those 4 fish A HUGE BIOLOAD like everyone is saying..give your heads a shake guys.

I would also try washing down the frozen food you're putting in your tank with pure water...might be getting phosphates or nitrates, or who knows what else from that. I found cyclopeeze was contributing to a huge problem in my tank. since I quit feeding it, my sand's been cleaning up and my glass doesn't get dirty nearly as fast.

also, try to get some readings for NO3 and PO4.

do you have a filter running with filter floss or a sponge? if so, either replace them or give em a really good cleaning in some waterchange water so the bacteria doesn't suffer.

and POSSIBLY, how old are the lights on top of your tank?