You canNOT add another fish to that tank. 5.5 gallons is far too small for any more fish. Taking fish out, on the other hand, would help reduce your nitrates which would stop so much algae growing. Try increasing the number of water changes (always with de-chlorinated water of course) and maybe add an algae-eating shrimp though it may or may not help. Adding a small plant would be even better as it will use up the nitrates and help kill off the algae. Also consider keeping the tank in light for fewer hours. If it actualy has a hood and lights, reduce the hours of light to about 8. Even with plants, this should be enough. If it's near a window, move it away or just add some plants and increase water changes and see how it goes.