I had a heater explode in my tank in early December. My LFS is baby sitting my fish for me. We did a 5 gal water change a day for 10 days to get rid of the smell and hopefully the bad water. I have done several more changes since and the nitrates just won't come down. all of the other levels are fine, (can't quote numbers at the moment). We took out all of the filters in the lower tank, the bio media and the sponges and put live rock underneath. I bought a new external pump to increase the water flow. I have added carbon in a bag in the bottom and also some white stuff (can't think of the name) in a bag that is supposed to reduce nitrates.
The nitrates will not budge - there are snail flourishing, laying eggs etc, all the crabs are quite happy running around and there must be a couple of hundred tiny starfish that are very much alive.
The only thing we haven't tried is to replace the skimmer as mine very obviously is not working.