From some calculators that I have used, dosing a teaspoon in a 6 gallon tank raises the potassium to over 100 ppm; chloride over 35 ppm. Granted that is for 100% pure KCl; I bet you'd still have well over the limit of chloride/chlorine that is safe for fish/inverts though. The potassium doesn't matter a whole lot (until you get to crazy high concentrations), but the chloride is probably what did it. You probably would have been okay if you added the KCl to a bit of your aquarium water and added some Prime or other dechlorinator before adding to the tank.
I could be thinking about this wrong or missing something else, but that's all I can think of off-hand at the moment.
Last edited by ngrubich; 09-07-2013 at 10:37 PM.