tank mate for a lionfish

[TheMikey]

I would STRONGLY advise against putting any other fish (especially predators) in your 30g tank. Unless you get a big protein skimmer, you are going to run into severe problems with your water parameters unless you're doing massive water changes with some frequency.

Also, minimum tank size for Picassos should be 75-80g. 30g is just too small. Same thing with the lion, I would suggest seeing if you can sell it to someone with an adequately sized tank (45g minimum... ideally 60g) instead of keeping it cooped up in such a small area.

If you're dead set on keeping a lionfish, there are several dwarf species that will be fine in a 30 gallon set up.

First things first, get a protein skimmer.