HtH and its sister company (PoolLife? or something) made a cal-hypo tab, first one in a plastic tubethat dissolved, then one without the tube.
The tubeless ones were pure garbage, becoming a pile of mush in less than a day. Never got hold of the tube ones but they were reputed to work much better.

But there are two caveats:
1) Cal-Hypo raises the calcium level in your pool so you have to stop using it at some point, plus it can be a bit alkaline, raising your pH
2) Never, ever put the tabs in a chlorinator or floater that had tri-chlor or di-chlor in it, and never let bleach touch cal-hypo! Very Dangerous!