My tank is about 5 months old. Everything is going great but I have been finding my crabs dead. I had 8 scarlet crabs and now down to 3.my parms. are Salinity 1.025 p.h. 8.2 ammonia 0 nitrite 0 nitrate<5ppm Alkalinity 9.4 calcium 420ppm Phosphate 0. I have 2 clowns,2 cleaner shrimp, number of snails, yellow polyups, frogspawn, green polyups, and a few zoas. Why are my crabs dieing? Thanks