I actually agree with most of what Eric says.. except i found they prefer well aerated water, and in general a standard water flow is just fine.. Mine will actually jump up into my HOB filter to get better aeration if the filter starst to clog up. I agree thought that your main problem is that your keeping one of ime the most docile fish with some very aggressive coparitively tank-mates. I keep my baby atm in a planted community includeing some clown-loaches, blue rams, glo-light tetras, albino cory cats, and a starry night eel ( not reccomeneded and the tank is for the juvi fire to get large enough to go with some larger tank-mates who atm might harass him to much) My last fire eels outgrew a 75 in under 2yrs.
You may also want to look at cutting back or out the salt. none of your fish are brackish.. and while salt is great in inhibiting negative bacterial growth ect I also found it tends to "stunt" biological bacteria media growth. this may also be causeing an issue. you also didn't mention how often or how much water your removing when doing water changes, this can make a significant difference in fish as well. nitrates over 20ppm can also cause negative effects to fish... lol as you see the list can be long of what could be done to either improve or stunt fish.
I would say at 7-8" though to try adding nightcrawlers to his diet or soak a sinking pellet like massivore in the bloodworms when you defrost them and let them soak up some of the skunkyness that is so attractive to your eel and see if he'll take the pellets, spineys taking pellets is uncommon but def worth the effort if you can get them to start takeing.
