Well 1st, what if your definition of a predator tank. Length is actually the most important dimension when it comes to eels, a full grown snowflake can average around 30". Gallon wise that tank could accommodate many species, but the square dimensions will restrict some, larger fish will want more length. The valentini puffer is a notorious nipper and I am hesitant to recommend any lions, and no anglers. You could consider a scorp on the lines of a Scorpaenodes brasiliensis(aka barbfish) or other similarly sized scorp that you may find available. Any larger predator with a large mouth will eat anything small, and those dimensions will limit you to larger fish. Without the puffer you could do dwarf and medium bodied lions, a properly sized angler, and other assorted scorps; that's my idea of a predator tank. If that puffer becomes a nipper it usually ends badly. Lions and anglers get stressed and feeding becomes even more difficult for species which are already challenging to feed. Injury to an angler can turn bad quickly. Or they end up on the end of a lion spike.