How can I lower the high nitrate levels in my saltwater tank?

I have a 30 gallon saltwater tank with very high nitrate levels. The nitrate levels read over 200 on the test sticks. The only animal in the tank is a small 2 foot long green moray eel who seemed to be in good health until recently. He has stopped eating but has gone through a non-eating phase before. There are also a few snails. I have just done a 50% water change which surprisingly didn't help at all. I have also tried many water solutions and bacteria that are supposed to remove nitrate but none have helped. Even the "all-gone" filter sacs haven't worked.

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