I don't think this applies with the Cal-Hypo tablets of HTH products, but I believe their Tri-Chlor (PACE Tablets I think they are called) are now a lower chlorine than they used to be - and they also have a copper based algaecide in them - which can lead to problems long term.

As always - check the labeling closely before buying.

Also - if you have a heater on your pool - it's probably not a good idea to do any chlorination via the skimmer - bad for the head exchanger.