QT is pointless now but it would've prevented it as you've learned. It's all good so let's move forward. Just treat the main tank. REMOVE THE CARBON FROM YOUR FILTER FIRST !!! It will take any meds out that you are using. It sounds as though you're treating based on symptom only. To truly identify if it is flukes you really need to scope them. Prazi will work fine, just follow the directions perfectly. Also it is somewhat difficult to overdose it. If after the treatment period (2 weeks I think) the other choice is Quick Cure. It contains Prazi and Metro, but I would wait at least a week in between with lots of WC
's. The 15 would work for the two 3" as a temp solution but would not be my first choice. I understand that's all you have to work with but if you get a chance the next time one of the big chain stores is doing a $1/gal
sale, grab a 20 or preferably a 30.