Water changes, reducing feeding, a protein skimmer, and a refugium all help with nitrate reduction. Most of the time nitrates are high because of feeding and/or overstocking.
How much and how often are you feeding?
Well that's stuffed. I would suggest a 50% water change and then another 50% in a day or 2. Cut feeding from a pinch 3x a day to 3x a week. I'm not sure how often the eel needs to eat. I'm sure somebody will chime in. I am pretty sure it he gets hungry he'll make a snack out of almost anyone in there. They are opportunistic feeders. I would look to rehome a lot of the inhabitants for the tank is quite small. I know it sounds harsh it's for the best intrest of your fish