I just went to Petsmart today and got Instant Ocean. I have a few questions about how I should use it. First of all, because I have brackish fish, how much should I use? Second, are any of the fish in my 20 gallon, such as Marbles, intolerant of brackish conditions? And third, though quite off topic, Ying just got a lot worse with the finrot on his tailfin. The only things wrong with the water are the alkalinity and ph. It is acidic in the tank. NOTHING else is wrong, not nitrates, nitrites, or even amonia. I am going to do a water change tonight and if given answers soon enough I'll use Instant Ocean.