nice tank i dig the black sand
it gives such a contast to the color's in the tank
as far as it posioning your tank i know a few people who use it and do not have problems
and for the buffering effect's of araganite they are minor becouse it needs CO2 to start that process
but you may not be having any problems since there is not too many large corals in there but test kits will tell you where you are with your ph dkh and ca