Freshwater Aquarium advice needed! When should I add fish?

If you don't have any fish you prob won't see a nitrite or ammonia change; this is caused by their waste. Also; you are doing correct water changes which also is also keeping down you levels as it's suppose to. I like guppies; however they breed quickly and are a hardy fish. Goldfish produce a lot of waste and can grow large; so you can only have a couple. Every fish has a draw back; bettas fight and may attack other fish. Research and waying the pros and cons is the best to see what u are capable of handling. Also I would look into small algae eater. Some can grow large and produce more waste than they help clean; but i think ur ok with a 20 gal. Good luck.