I know I'm a little late getting in on this but as stated earlier 20 isn't bad for fish only. Also, doing two 15% water changes you can't expect a huge drop. I just brought my nitrates from 160++ to about 20 and I'm happy now. It's now a matter of keeping up with the changes and trying to slowly get it lower. I've also added something like Bio-gen to my filter to try to absorb some of the ntrates but haven't noticed much change. Really it was just water changes and cleaning out some of the old gunk. New fish should be able to handle nitrates of 20 as long as they are aclimated slowly. Much higher and I'd be concerned about adding livestock. But the answer really is water changes.