I need some help, my wife’s tank with her angel fish, barbs, and tetras arendying, over the past two days 2 bleeding heart tetras and 2 angel fish died, I noticed at first they were all swimming to the top of tank looked like they were gasping for air, I have an overhead filter and also air bubbler so I think they have enough air, and before the tetras died I noticed little air bubbles all over them, as well as on the angels, they started swimming sideways up upside down, I have done 2 water changes. All I have is test strips the only thing that is off a bit is the nitrate. Any help would be greatly appreciated. My wife isn’t very happy, it is not a new setup we have had all these fish in there for over a year. 2 weeks ago she got 2 very small cherry barbs, and they died so I think maybe they had a disease or something?