I use morton purex all purpose salt. I make sure it is non iodized. It's like 6or7 bucks a bag. and ive been using it for years no problems. 1 cup=40gallons.
instant ocean, kent seasalt, and coralife are all made in the same plant!! same salt with a few minor adjustments. i use kent because if you read the ingredients then you will find that it is a bit more stable. instant ocean has always been a 4-5 star salt. good product!!
