With a 55 gallon you have a ton of options. What fun! Mollys, guppies, a gourami or several honey gourami?
What is “bad” about your water? Maybe knowing that would allow us to help better? Can you do to a fish store & write down fish you like the look of & we can help you figure out what could work together. Do you want babies or not (that will dictate sex choices for the fish).
Most any species are good with Platys; I have a betta with some Platys. And a H. Gourami as well. They all get along just fine and are seem to all be friends. Yeah, please clarify/define "bad water":...pretty please!