I bought a supply of these at Walmart a while back and run them through my Pentair R171218 Rainbow 320C as a backup. I'd say around the time I started using these was when I started having trouble keeping my chlorine levels up when on backup. I recently refilled my pool and started clean, and immediately have the same issue. Last year I had a supply of 1" tablets to use up, and I ran them through the same Pentair. Then I had the opposite problem, my chlorine went to shock level and stayed there. So the Pentair works and clearly there are different formulations of trichlor that erode at different rates. These do say "slow-dissolving". Is that my issue? I haven't seen a single thread where it is recommended to experiment with different tabs.