One 6V battery discharging really fast

We are all just guessing that you may have a shorted cell you won’t know for sure until you check it either with a hydrometer or multi meter like wopachop described. That is how I do it I have not had good luck with hydrometers. It is not uncommon as batteries age that some cells read less than others. Equalization usually fixes that. I have brought mine back several times using that function on my charger. Try the test first, if it is totally shorted it’s time for new batteries if one cell is low try to equalize it and see if it holds a charge.