Hi everyone. I don't understand why, but it seems as though my crabs aren't surviving my 3 month old tank. I have had 3 crabs die in the past 4 days. I know copper is the first red flag, but i have poly filter installed, so it can't be that. Calcium is at 440ish? I'll check it again in a few to make absolutely sure. Nitrites 0 Nitrates are a little high, maybe 20 Ammonia 0 pH 8.3 Salinity is 1.024 temp 79 Tons of algae to eat, and I feed the fish once a day, so they have plenty to eat. My snails are thriving and eating more than ever. I had an emerald crab looking awesome, then dead the next day. No slow deterioration or decrease in activity. Any ideas?