Can I add a (1) blue gourami with my 12 tiger barbs?
No. I’m sorry but you are already seriously overstocked. I expected to see a tank size of 55 gallons minimum for twelve tiger barbs. They do not have room to grow to their adult size in your tank. 6 would have been plenty. The guy at your pet store lied to you, or was lied to himself, in order to sell fish. Cycling with fish can be dangerous to the fish because until the good bacteria reach the stage where they can handle the fish waste, it builds up in the tank. Ammonia and nitrite are very poisonous to fish. Nitrate is harmful as well, but not nearly as toxic. Nitrosomonas bacteria convert ammonia into nitrite. This is the first part of the cycle, if you have a test kit, when you see the ammonia drop to 0, the first part is complete. It still needs ammonia to live, but will convert it as quickly as it is produced so there should be no READABLE amount. Nitrobacter bacteria convert nitrite into nitrate. Once these are present in sufficient quantities to handle your nitrite, nitrite wil
