I am new to saltwater (29 gallon with live rock and live sand) Protein Skimmer and circulator. After one month, I added 4 scarlet hermit crabs, turbo snail, Astral snail, 1 nassarius snail and peppermint shrimp. The following week I added 3 blue hermit crabs, 3 margarita snails, 2 cerith an emerald crab and green brittle star. Over the week following I lost the peppermint shrimp, 3 margarita snails, 3 scarlet hermit crabs ? 1 blue hermit crab and the emerald crab. The water conditions are nearly perfect except I have small elevation of Nitrite at 20 ppm and specific gravity of 1.027. I preformed a 20% water exchange today but I can't figure what caused their death. Any ideas? Thanks