which is also why you cant get rid of it with water changes as fast as your wanting ,like fishytime said your tank is constantly playing catch up and since your never really removing the problem they never go down,if it does more nitrates are produced faster than you can change them out
i would be iffy on having 20 fish in a fully established tank let alone one that hasnt made the 6 mth mark yet.size is a factor of your fish but between a big fish and a lil clown a lil clown still excretes lots of waste may not be as much but waste is waste and your system needs to adjust.thats why i use the rule of thumb of no more than one fish a month ,not that i would buy a fish every month lol because it gives your system a chance to even out and thats your goal is to get things stable, once stable you allow time to pass then you know youve reached "equilibrium" once youve reached that stage you can get another fish but your system has taken a change and now needs to stabilize again and find equilibrium.
personally untill a system establishes(up to a year sometimes more)you should stick with a low amount of fish.20 fish must be hard to feed without feeding the tank and i bet your getting alot of your nitrates there too
go slow milad buddy sit back and wait for your system to establish, youve already got alot of fish and the damage is done so keep up with your routine regular water changes light feedings stop adding stuff to your tank as its not needed (yet) and possibly think of getting rid of a few fish and buy some coral
in time your going to have trouble with all those clowns, if they get along now fine but when maturity hits and territorys are getting established the fights are gonna break out and if theres alot of tension your going to end up with disease, and you dont want that.