Tri-chlor tabs are fine to use IF you know what they do and know that's what you need. They add chlorine, are EXTREMELY acid, and add 6ppm of CYA for every 10ppm of chlorine. For a very alkaline pool with no stabilizer, they are great! (New masonry pools frequently have high pH and need lots of stabilizer). For a very acidic pool, or a pool with high stabilizer levels, they are the worst thing.

Di-chlor, while not as acidic as tri-chlor, add even more stabilizer--about 9ppm of CYA for every 10ppm of chlorine. If you pH is in the middle but you need stabilizer, di-chlor can be an excellent choice.