why Prazi? its primarily a de-wormer..
if they are flashing, and you don't see ich parasites, it could be simply a water quality issue that is affecting their slime coat.
a few extra water changes and some salt might be all that you need to clear that up.
and it wont put your biofilter bacteria at risk..
(I have had a nitrite spike before when using prolonged doses of meds - lost some really nice brackish archers that I had). ever since that bad experience I am nervous I guess..
good luck!