But did you keep the triggers to adult size? I don't know of any species that wouldn't be cramped in a 40 or 55.
I have been growing out a baby niger trigger in my 55. At about 4 inches TL, it's ready to move to the big tank. That is a fraction of its adult size.
A 40 FOWLR would be a nice size for several aggressive fish, but triggers would not be on my list unless you were growing a baby up for a much bigger tank.
If you plan to keep the fish for just a year, and definitely can find a decent home for it, then get a single small (2.5-3") undy, niger or picasso. IMO, you will have NO MORE than a year or so before the tank is outgrown. It really isn't fair to the fish to keep it in excessively cramped conditions.
What kinds of fish do you like? We can suggest stocking lists of animals that could spend their adult lives in a tank that size. Do you prefer fast aggressives or slow aggressives (ambush predators, mostly)?
For example, you could center a tank around a sharpnose puffer and compatible species, or a dwarf lion and compatible species (but not both in the same tank).