Question about red platys.?

I love aquatic life. I have had a saltwater aquarium for some time now. They are great fish and very clean too. When I was young, I had gold fish. I am interested in getting a freshwater tank as well as my saltwater one. I think the red platy fish is a beautiful tropical freshwater fish. Every time I got to the pet store to eye them up, I notice they are always pooping. My saltwater fish (damselfish and clownfish) are very discrete when they poop. I see them go once a month ( I know they go more, but I just don't see it happen) and it is never dirty in their tank. I know they have a lot more filtration with the live rock and sand and the filter. So my main question would be, do platy fish defecate a lot, are they dirty fish, how can I best keep this tank clean if I choose to get them and are their any invertebrate that can live with them to help keep it clean beyond the means of just a filtration system?