Actually, a
P. volitans in a 75 gal is pretty tight. Basically, a volitans will be a minimum of a 12" cube in the tank, counting pectorals, so imagine that fish trying to turn around in an 18" tank that has LR taking up some of that 18" depth. Additionally, IME, lions that don't have sufficient room to maneuver tend to be a lot more "passive" and tend to perch a lot more than fish that have some room.
However, all is not lost...if you look into getting a P. russelii (Russel's lionfish), often incorrectly sold as "red volitans", it should fare much better in the space of a 75 gal as they only end up as about a 10" cube of fish. Many folks can't even tell the difference between the two fish, and their care and habits are the same as for P. volitans.
Generally speaking, triggers and lions don't mix well, with the exception of the planktonic feeders (genera Xanthichtys, Melicthys, and Odonus), most of which get too large for a 75 gal, altho you could probably get away with a blue throat (X. auromarginatus) in a 75 gal. Likewise, large angels and puffers aren't compatible with lions.
Squirrelfish/Soldierfish make good lionfish tankmates, as do marine betas. Other Scorpaeniformes do really well with them, such as Scorpaena brasiliensis (red barbfish), Inimicus sp. (sea goblins), Rhinopias sp.(frondosa or eshmeyeri), certain eels (zebra, SFE). FWIW, I wouldn't keep a ray in a 75 gal on a bet, even the smallest ones (we're talking 240 gals min with lots of open space).
The key is to get the lionfish well-established before adding any tankmates, and make sure said tankmates can't be swallowed, so you're looking at a fish that is at least 3/4 the size of the lion and not slender or torpedo-shaped unless they're larger than the "3/4 rule".
Here's a pretty decent article on lionfish that should help:
http://www.lionfishlair.com/careguides/lionfish.shtml
If you have any additional questions, there are lots of sharp folks here.
Sorry for the length of the post.
