Hello all. I have a 20 gallon freshwater tank with plastic decorations that finished cycling about 3 weeks ago. We initially started off with 3 tiger barbs, then this week we got 2 more plus a pink kissing gourami. Yesterday I lost one of the original tiger barbs after he started acting strangely (getting sucked onto the filter intake, swimming on his side, sinking to the bottom and swimming in weird circles to the top). This morning, it seems that another tiger barb has isolated himself at the top of the tank, and another is doing the 'circles of death' and behaving oddly like the first one did. The other fish appear to look and act normally. There is no spots or discoloring on them or in the tank. My tank parameters as of this morning were as follows: NO3- 30 NO2- 0 PH- 7.0 KH- 40 GH- 120 Ammonia- 0 I am running a penguin Marineland filter and have a round bubble stone in the tank. I did a partial water change yesterday and also added 2 tablespoons of aquarium salt to the tank. Should I isolate the 'sick' barbs? Could they just be bad stock from the LFS? I don't want to contaminate the whole tank if they are sick but the other fish seem fine for now.