Does everyone cut the bag open to get the right amount for their aquarium? On their website it says not to but I want to see if other do with no ill effects on the tank.

i do. i cant fit that big bag in my filter. so i take it out, mix it up real good, then put it into pouches i got from the LFS. ive had no issues.. chemi pure is the best thing ive ever bought for keeping the tank stable.