Bad Sand....
Used Sand=Crashed Tank....
Take it from me dude, I was like you, I needed sand so I used someone else' sand and had about a 3" sand bed when when my tank crashed one day for no reason luckily the fish remained ok, but my inverts and corals all perished and i had to do a complete tank makeover,(new sand, water etc..) which if anyone remembers me (which i am sure they do lol) knows my tank is slowly, i am talking slowly recovering still to this day about 2-3 months later....but i am not jinxing it just yet =0
It was a hydrogen sulfide(sulfate) reaction in the sand....(black or gray spots that show up deep in the sand, youll know it by the rotten egg smell... but i am sure if you wanna know more about it, there are members on here who have some crazy knowledge on that subject....
Do yourself a favor and get new sand, and a good ro/di setup, as well as a bigger skimmer LoL ....Words i am living by.....