I kept a Gymnothorax Undulatus (Undulated Moray)for 26 years, so take what I say with the proverbial grain of salt - I'm still in the learning stages!
When I got Hannibal, he was ~18" long and maybe 1 1/2" diameter. Cute little guy. That didn't last long. Within a couple years he was just short of 4' and as big around as my upper arm.
I kept him in a 150 long (72x2x21) with a 50g sump.
He was a voracious eater. I finally gave up trying to keep other fish with him becasue he'd eat anything I put in the tank. XXL sailfin Tang? lasted all of 2 weeks. 16" Emperor Snapper? I removed it for it's own safety. A 18" golden puffer? Yup tried to eat that too. 3 palm sized filets from Kroger's seafood counter weekly kept him happy. Once well fed, he'd come to the top of the tank to be petted. He felt like wet, slightly slimy leather.
All that eating made for a huge mess. Even with about 175g of total volume and 30-40% water changes weekly it was impossible to keep the nitrates low enough to even be tested. In 2000 I got married and we moved into a new home, so I made a fish room in the basement and added a 120g DSB (6+ inches of sugar fine sand) refugium with ~ #200 of live rock suspended above on a PVC rack that was finally able to keep up with the nitrates. So now I'm at ~300g total volume. For a single fish.
Eels are the ultimate escape artists. If there is an opening that it might squeeze through, it will. I started with eggcrate covers, and after the fist time I had to pick up an angry eels off the floor, added large pieces of rock to all the corners. He managed to escape that a few years later. I found him on the floor dried out and thought he was dead, but when I nudged him with my foot he moved so I picked him up and put him back in the tank. Other than a nasty fungal infection he was none the worse for wear. I ended up making a canopy out of 3/4 plywood that was so heavy it was difficult for me to lift, but he never got out again!
As Eel's go, the Undulated moray is one of the larger (and not often kept) species, due to their appetite for fish. The Columbus Zoo Aquarium gave me an emphatic NO! when I tried to donate him so that I could turn the tank into the reef I had always wanted it to be, so I kept him until he finally passed. Be aware that many eels, if kept in reasonable conditions, can live for decades.
HTH!
in his younger years - he was probably 24-28" here