Help, my fish keep dying.

We have a 20 gallon tall salt water tank, I know it's small, but I have no room for anything larger. Approx 5-10 pounds of live rock and 2-3 inches of sand. There are 3 snails and a scarlet crab that is still alive. Everything was fine, we had one clown fish and we added a dotty back and both died within a few days. We checked the water at the local fish store and the nitrites were high so we let the tank cycle for a month while doing water changes. We had the water re-tested and all the levels were great so we added 2 new clown fish one died in three days, we bought one replacement and both were dead in a week.

The death has been on going for the last two months. we've started buying cheaper fish, damsels - they usually last 2 or 3 days, the longest lasted a week. We never add more than 3 fish at a time. I feel like Darla the fish killer. The water tests all come back good. the snails and crab are all doing good.

We have grounded the tank, added a heater, added a wave/skimmer thing (alternating turning it on or off, a few of the fish seemed scared of it) and a bag of stuff - Ckear FX Pro- to put in the filter recommended by the LFS. We haven't done any water changes in a few weeks, everything was dying whether we did it or not, so we didn't see where they were helping.

any ideas on what to try next? I'm getting discouraged and considering fake fish.

thanks