There are three things that jump out at me right away when I read through your first post.
First, you have too many fish of too disparate types. Inch per gallon does not even come close to applying here, your cichids need more room to establish territories, and this means having things like caves for them, which takes up space.
Second, insufficient water changes. You should be aiming at a minimum 50% per week. Changing more water will
not stress the fish. You don't need to do it all at the same time, two back to back 30% changes will be just as good. The closer your tank water chemistry to source (tap) chemistry, the better. People who claim that changing lots of water per week stresses fish don't change their water enough. What stresses fish is the buildup of organics in the water from fish wastes, you cannot measure these, but they're there.
Third, your nitrAte level is not 0, it is impossible. Even in an understocked tank, which yours is not.
Don't look at overstocking as a failure, we've all done it, I once had a 10g tank with 3 swordtails (+fry), 3 albino cories, 2 plecos (yikes!) and a declining number of tetras, first neons, then king blues. Look at it as an excuse to get another tank.
