Some fish Dying others Thriving.

Hello! I cycled my tank for about a month. I tested it and it came back perfect for all parameters. PH 7.6, Ammonium 0 , Nitrate 0, Nitrite 0. I specifically had it tested a few times to ensure it was good. Then I bought myself 3 glofish, 3 Zebra Danios and 3 Tetras (I believe phantom tetras they were called but not 100% sure). My temperature is set to 24 C. I bought them just this past Sunday and since then have lost 4 of the nine. The first night I lost 2 fish. There was no signs of illness and it was one glofish and one tetra I lost. I really tested the water...It came back perfect. I asked what else it could possibly be and he said maybe a low oxygen since my fish had kinda been handing around the top half of the tank. I purchased an airstone and have it on the side opposite the HOB filter so that the whole tank can get a bit circulated. The deaths stopped for a night. And the fish seemed great. Then the next morning one of my tetras was stuck to the filter dead. And another Glo fish died before my eyes. Now the last Glo fish and tetra are not looking to good. However the zebra danios are doing great and within just 4 days they had a bunch of eggs that have hatched. I have baby fry inside my gravel. I think at least 20 I can see. My question is why are only the zebra danios surviving? And also is it possible they breed that quick? I'd like the fry to survive but I don't want to kill them when I clean the gravel and I know the adult danios now skim the surface of the water and eat any they can find. I'm not sure how many they have eaten but at least 10 I think. About 10 left more or less. Maybe more I can't see. Why would only one kind of fish be surviving? Are they just more hardy? What is left to do? I'm buying some purichem to replace my carbon. I've been doing every 4 days a 30% WC. It's a 40 gallon tank. I want my fish to live and i don't want them to keep dying

How is everyone doing? I have never kept zebra danios, but I do know they are deemed to be quite hardy. With a 40g, I can't imagine too many were added at once to cause a problem? Perhaps they were not doing well to begin with? Curious about any updates...

So it's been a month now. And 5 of the 9 died. But 4 remained. And a bunch of fry are much larger and hiding well from the adults. I think it was a combination of too many too quick in addition to low oxygen. Guy at the pet store recommended a bubble stone and none died since. It's odd to say the least since bubble stones don't add much oxygen but it was enough to save my fish

Well it may have been timing as well. Fish change hands many different times by the time they end up in the LFS and in your tanks. Think of the trauma of a fish most likely raised in Asia, being airshipped to a transshipper, then put into a wholesaler tank and eventually in a local fish store. It is no wonder that most fish deaths will occur in the first few weeks at the LFS. If you have not been monitoring the fish, and you purchase a fish in the vulnerable stage, you are most likely one of the statistics.

Sorry for the drama, but so true. The water is not the same, the shipping is more than most can handle. Don't blame yourself for the unfortunate deaths. The best scenario is for the LFS to isolate and quarantine for at least 4 weeks. This will never happen since they will lose too much in their stock so you end up with weak fish and unexplained deaths.