We did this as well. Made the mistake of listening to the LFS staff's advice. Well they were completely wrong. We went to another store, received better advice and decided either to spend some money and keep this awesome creature or return it.
Well the crazy fish lovers that we are, we decided to keep it. This moray eel is about 14" long and is now in a brackish, 50 gal tank all alone but seems much happier.
We set the water temp to about 75 degrees, the salinity is set to 1.010ish. We have added a cave to his new home for him to hide in and so far so good after 24 hours. And the substrate is pool filter sand. The same stuff we use in our 90 gal cichild tank and in our 70 gal odd ball/community tank.
I'll post again in a week or so and give some updates.
My BF wants to get some Archers because they spun pretty cool how they spit water to make little critters fall in the water to eat. Sounds like fun but a lot more work!