Threadfin rainbowfish disease

[dpstark2]

Hi,

This is my first post here. I am posting because I recently lost all 8 of my threadfin rainbowfish. I have several tanks, but this one is a 125gal tank with a variety of fish. It is primarily clown loaches, bosemani rainbowfish, cherry barbs, and odds and ends from other tanks. The threadfins were added a while ago. I can't tell you exactly how long, but they were all doing fine for several months. Then I noticed one sort of stuck at the top having trouble swimming down. I'm sorry, but I never took a picture. At this time, I also counted closely and noticed I had already lost at least one. Over the course of a week, I saw two more fish do the same thing. I started treating with antibiotics (Maracyn) at this time. After the treatment, I ended up with 3 left and they seemed OK.  Those three went over a week with no issues. Unfortunately, I came home to find two gone a few days ago... and the last yesterday. 

The tank parameters are: 7.6-7.8pH, nitrates usually ~50, no nitrite or ammonia. Hardness about 6-7 degrees and buffer 2-3 degrees- basically my tap water. The tank is at 80F.

I really wish I had a picture for everyone, but I don't. The fish showed no signs of other issues. They would just get sort of stuck at the surface, unable to swim down. Eventually they would end up nose-down and have sort of a hunched over back, and then pass. I recently read it might have something to do with food, so for reference I feed the tank a wide variety of food, but the only stuff they ate was crushed up extreme flakes and extreme nano pellets. And I did observe they would all eat... until they got stuck at the surface and really couldn't any more. All 8 were roughly full grown when I got them, so age isn't out of the question - but I find it odd that 8 would go within a few weeks of each other due to age.

There's nothing to be done now, but I'd really like to know if anyone knows what happened. And if it may resurface in other species in this tank. I'd hate to lose my bosemani... they're big and I've had them for a long time.

Thanks for any help.

[Colu]

Not a lot to go on hanging at the surface can be caused by low levels of desloved oxygen as no other fish were affected is odd so I don't think that's the cause did you any rapid breathing listlessness spitting food out loss of appetite @dpstark2

[dpstark2]

Thank you for the reply. The fish were eating consistently, and I never saw them spit out food. The tank has two air stones running and the fish were always nose down at the surface. They were also still quite active and seemingly energetic. They would try very hard to swim down a few inches, then pop back to the surface. They were never listless or lethargic until I believe they were just too worn out to continue. I don't have a lot of experience with diseases in my tanks. I'm sorry I don't have better information.

[Colu]

Usually when fish are having trouble swimming down and then floating back to the surface it's swim bladder issue it's very unlikely to affect all your rainbow fish it's usually the odd individual nothing else is jumping out at me @dpstark2