Sinner's got some great questions. Please answer all of them. Also, what filter do you have in your system. I'd also take a sample of your water to your LFS, and have them test it. Your results could be off.
i know this is not the problem, but your PH should be between 8.2-8.4, so after you get the nitrates under control, i'd suggest raising that before you try to add any other fish.
Well, the death of all of your fish is probably the reason for the sky high nitrates. You've probably re-cycled the tank. If your LFS comes up with the same results, you should continue to do 25% water changes, weekly. That should bring it down. Make sure you also vacuum up any leftover food, or anything in your substrate (even if this means losing some sand).
You could put your LR in buckets with new water (with good levels), and start over that way. Add new substrate, new water, put your LR back, and re-cycle the tank from scratch. If you have CC, this would be a good time to change to sand, IMO. Just another option. Before you do anything, though, have your LFS test your water.
Jenn