Giving Up.....

Bacteria are pretty tough. A temperature high enough to kill all of the bacteria would probably have killed the eel and crab.

Here's my 2 cents:
I think you have two choices: 1. Start all over and try again or 2. Take a series of steps to make this a functional system
After having the tank for a year, I would go with the latter.
-- reduce the nitrates.
-- Get rid of the eel or feed him less (NOT goldfish, which don't have the nutritional value he needs).
-- Get a skimmer.
-- Do a series of water changes over the next 2-3 weeks. Siphon the debris off the bottom of the tank. Change 20% of the water (6 gallons for a 30 gallon tank) at a time. After six water changes, the nitrates should drop from 160 to 40. After five more changes the nitrates will be around 15.

This is going to take time, and it's going to take money. If nothing else, the cost of the salt will be significantly more than you are accustomed.

Then comes the hardest part: re-evaluate what kind of a tank you would really enjoy. Then, don't buy anything that is not necessary for that vision.

Good Luck!