My boyfriend's parents have a 20 gallon tall tank set up. It's got 2 neon tetras and a bigger rounder fish his mom didn't know the species of. They found out I've started fish keeping and said I could keep some fish in it if I wanted to keep it clean. It's cycled and has a heater and filter system. The big problem with it is that it has a ton of algae growing in there. The glass and gravel are covered in a thick layer of green. The fish are all doing pretty well in it, they look healthy, but the tank just isn't attractive. How should I go about cleaning the algae out and introducing new fish without stressing out the current residents or having to cycle the tank again?