Clean up crews in general can be hit and miss. They (snails and hermits) can just up and die for various reasons. One reason being the lack of stuff to actually clean up (ie, food). Another reason is competition. It's a hard knock life being a snail or hermit crab scrounging for leftovers. I guess what I'm suggesting is that the die off of your CUC could be unrelated. Hang around the hobby long enough and you'll hear repeated frustrations of people having troubles keeping a clean up crew. In the end your tank will sustain whatever clean up crew it can. You can buy all the snails and hermits you want but your tank will only be able to sustain a certain population which may or may not match what you've actually purchased (usually not).
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