20 gallon fish tank fishes?

It is almost over crowded but you shouldn't put the mollys in with the plattys because their is a chance for them to cross breed I have a 50 gallon with only 5 fish in it my male mickey mouse platty cross bred with my female red velvet swordtail and now I have a male mickey swordtail that again has impregnated one of my female redvelvet swordtails