Found the molly. apparantly (s)he was smaller than I thought. Or the mini rapids filter tubes were larger than I thought. After my hornwort crumbled and all the leaves got stuck to the filter intake, I took off the intake's screen to clean it while leaving the filter running. He must have gotten curious, swam up the tube, then not been able to reverse. I think the tank already crashed pretty much. the water was VERY cloudy this morning, so I did a 50% water change and filled it up with dechlorinated tap water to find that the water was still cloudy. Thinking the filter must be clogged (Rapids minis are notorious for this until they're modded which I haven't had to do yet), I pulled the filter apart and found the molly decomposing in the filter floss.
so now I've tossed the filter floss, rinsed the bio rings in water that I hope was dechlorinated (I'd put Prime in a full 5 gallon bucket of tap water 4-5 hours before and used that water) and put the canister back together with new filter floss. Now I just hope I haven't eliminated too much bacteria.
Since the tank's at work and it's a 4-day weekend, I'll have to wait anxiously to see how it's doing. I sure hope it levels out, clears up, and the plants and other fish haven't suffered too bad.
Good thing it was a molly and not something more expensive! (not that it's ever fun to lose fish, no matter what they cost....)