How do I clean my fish tank when there is still fish?

I got a fish tank with fish from a friend who is moving and didn't want them anymore. The problem is they let the tank get pretty nasty. There are 4 fish and and algae eater in there, but there is tons of poop in the pebbles, or gravel on the bottom. We have a few fake plants in there, and the tank has a filter, heater, and light. How do I go about cleaning it, and getting the poop out without too much stress on the fishies? Thanks!