Help with a dirty goldfish aquarium?

My goldfish seem to be pooping alot and everyday when i use the special fish scooper to shake the fake plants the whole aquarium fills up with poo even though the filter sucks it right out. I have a 50 gallon tank with 3 goldfish in it. I feed them once a day. How can I stop them from pooping so much? Also why do they have an obsession with going under a little ornament bridge that I have? Thankyou for your answers