If a poison dot adds 10 counters, you have to remove those 10 counters before the spell will be removed. Each cure type spell removes a certain amount of counters. Generally, the higher level the cure spell, the more counters it will remove. I.E. If a spell puts 50 counters on you and you're curing it with a spell that removes 10 counters, it will take 5 casts to remove it. The only exception to this is the bard cure lines (aria of ascetiscism for poison/disease and aria of innocence for curse). The removal effectiveness (number of counters removed) goes up depending on the modifier on the instrument they are using. As for stacking, generally, spells from a single line will not stack. If you have a level 16 poison dot and get the upgrade at level 24, casting the level 16 one then the 24 one will tell you that your level 16 one has worn off. BUT, 2 of the same spells cast by the same class of the same level WILL stack. A little confusing, but with a little communication with your group mates before a fight, you should be able to work out who's casting what and see what's stacking according to your wear off messages.