ok so i have a 20g with 2paradise fish 2platies 2cories
what else can i put in i want more platies or colorful fish and i know i need more corys also i have some brown spot algae what fish will get rid of this
A 20 G is pretty small. So, I'd get two or three small corydoras, like the Albino kind and call your tank done as far as fish. However, you could still put in some plants provided you have the room.
Speaking of room, at your first opportunity, get a 55 G tank and go to work. There's really no limit on what you can do with a larger tank. I had small tanks when I first started in the "water keeping" hobby, thinking I couldn't handle a large tank. But, the large tanks are very forgiving of mistakes in management, a lot more so than the small ones.