Your chemistry is pretty good but the problem is most likely in part due to the NO2
reading. That should be zero at all times. With NO2
present, your NO3
's are most likely elevated as well. Do you know what the nitrate level is?
What do you normally feed the fish, how often and what is the primary filtration source?
What time of day was the ph
tested? 8.0 is a little on the low side but may not be a concern if tested early in the day.
I would caution you on the horseshoe crabs as well. They can be quite destructive due to their clumsiness and can quickly devestate the fauna in the sandbed. Your snail population is a little on the low side but if your other "critters" are keeping the tank otherwise clean and the algaes are controlled you may leave well enough alone...