I had heard that 1 inch of fish per 5 gallons was a good rule of thumb, rather than going by number of fish (which goes back to the "everything poops" and bigger fish poop more idea).
I have a 26 gal
front and a 18 refugium, and I'm pretty sure mine is maxed, too. It's a pretty stable, established tank, but just before vacation last week, I added a school of 3 chromis, and I wish I hadn't. I think that pushed the limits of the tank. Seems crowded.
But worse, my nature-lover son brought home 3 hermits: 1 in a shell the size of my hand, and 2 about half that size. I put them in the tank, so they won't die overnight, but I think I'll take them to my local fish store ASAP.
Right now, I'm not sure what they'll do to the local inhabitants, or whether they will "drown" as I have read elsewhere. They appear to be okay, and very active, so maybe they're slow drowners.