6 fish died had clear bubbles first

Question
Hello, I got a new tank pretty big in size, I put four sickling in originally and they were all fine. I then put 2 new fish in and a couple days later I noticed clear bubbles on all the fish, then loss of color,then looked liked pieces of skin hanging off them and eventually death of all G.I. don't understand what happened I tried everything but I wasn't working,I went to PJ"s pet center they checked the water twice nitrotates were low first day so I put the conditioner in the next day the test came up good but they were still sick he said to put this salt like stuff in then said are you siphining the gravel? Unfortunately I had no idea you had to do that I got home ASAP siphoned the gravel and it was a very dark brown color So I guess what I'm asking is what did the fish have and how can I make sure it won't happen again and how long to wait to get more fish also how to make SURE that disease isn't still in the tank? Thank you so much I'm heartbroken

Answer
Ashley,
New tanks need to be cycled before you add any fish. The cycle process can take 6-8 weeks. During this time two bacteria blooms set in. The first is the deadly bacteria that can kill all the fish in the tank. Once that one is gone, the second bloom sets in. This is the beneficial bacteria that both the fish and the tank needs to be healthy. If you did not cycle the tank, chances are the fish died from ammonia and/or nitrite poisoning. When testing the water, the ammonia and nitrite have to be zero at all times. Nitrates no higher than 20ppm. I do not think that they died of any disease that would still be in the tank. I would clean the whole thing out and start all over. If you need help with cycling and what needs to be done, just let me know.