Quote Originally Posted by famousdavis View Post
What's the difference between calcium chloride dihydrate and calcium chloride? Is one preferred over the other?
Only difference is the amount needed to achieve the same rise in calcium hardness. The dihydrate has two water molecules attached to it so it weighs more so you need to add more. Whichever you can get is fine. I have found the dihydrate is more readily available and usually buy the Tetra brand in 50 lb bags. Dowflake and Peladow is usable but it does have a higher bromide content (although not as much as if you add a sodium bromide based mustard algae killer).