I would not have started with that many fish in a 20. IMO
, three danios would have been plenty to get the cycle done. Danios, platys, and corys are fine though...all pretty hardy fish. Not so for the otos. They die fairly easily and do not tolerate poor water conditions such as ammonia or pH fluctuations. Give it some time and your water should stabilize. I think I'd take a water sample to the LFS
and get it checked there to verify your test results.