cool water fish
I'd avoid more fancy fined goldfish as the comet may be more aggressive toward them. For an algae eater you could go for an apple snail, if you have no or dont care about your plants. A hillstream loach eats algae well and is cool water. A weather loach is also cool water but not an eater of algae. White clouds are great but may be eaten if your comet is big enough.
What temp is the water kept at?