Almost all the tabs are Trichlor now AND just recently a lot of them have copper in them as well. The dichlor tabs are around some places but not anywhere I have looked recently (and I HAVE looked) The difference? Ummm...I know this one but it escapes me. Two vs. three (di vs tri) chemical compounds/active ingredients - the chlorine, buffer (cya) and other ingredients like copper (to act as an algaecide) not counting the inactive filler crap some have. Thats my best guess off the top of my head, but I bet if we look it up on the site, its here somewhere. Chemgeek?