There are two types of Cal-hypo tablets that do not add CYA to your water and are supposedly slow-dissolving. Both are sometimes called "Duration".

One is wrapped a plastic capsule that you then dispose of. Get this type.

The other is not and will dissolve in a skimmer in a matter of hours, and block the skimmer...Guess how I know! Don't waste your time or money.

HTH used to make the plastic type but now makes the caplet. There is a sister brand that I can never remember the name of that makes the plastic capsule.

Again, that's what you want, that plastic-wrapped Cal-Hypo capsule.