Help replenishing my cuc

[cancun]

Hi all! Everyone I need so advice....!! I had 4 small snail...brittle star...peppermint shrimp. ..and a cleaner shrimp as my cuc....all of my snails have died so I was wondering how important is a cuc....? Also what about a sand sifting sea star? Sea cucumber? Some to really clean the sand....any ideas? Also I have had some red legged hermit crabs....but they seemed to be very aggressive. ..so I never got any more crabs....any thoughts on crabs? I have heard people say crabs have attackedvtheir fish....star fish....etc....any help would be greatly appreciated!! Thanks!!