The 5.5 died completely. I didn't realize it until I was removing fish and snails for the 40. Nothing but nasty sulfur in my face.
Soaked the sand and gravel in vinegar, lightly rinsed and then boiled the heck out of it for a couple of hours. Vinegar gets more caustic when boiled btw, something to think about. Rinsed it for half an hour solid (my poor back!) and put it back in the tank which had been wiped down with vinegar, then rinsed. Neither smell nor cloudiness when I sift the sand. Going to leave it with filter, bubbles, heater and substrate. Also boiled DW and set it on a shelf to dry out. No idea what to do. Maybe a breeding trio of class N Endlers? Some moss balls?
Do you know what happened to the tank?
You could do a trio of Endlers, even a trio of scarlet badis would work in that size tank. School of micro rasboras. Shrimp.