you said you just recharge the carbon??? by adding more to the old? or completely replacing? if so i'd try completely replacing it... i doubt it is a voltage issue by the way the corals are affected.. and all your levels are in line. I'd try a double check on all your water parameters including the salinity.
also.. how are your sps dying.. at the tips first than all over... or from teh base first than all over..
what are you using to dose your tank? I think the instant ocean is a mistake... Im thinking you need to switch back to reef crystals