I would cut down on the crabs,,,hermits, etc get a package with more algae eating snails.
Did you guys see what we did to find out what was killing your snails, crabs. We had to take away the things we had control over...the water parameters, feedings, new additions, etc and then we could go to the next level and see if you noticed any sites or sounds and bingo I believe we got it. Sometimes it takes more information, but that is the reason we ask so many Questions.
Good luck with the new CUC when you get it. Be sure to acclimate over a longer time both temp. and PH and water mix.