Sick corydoras, please help!

[cattyrat]

I have a 90L tank with, among other fish (betta, bristlenose pleco, some ember tetras, all of whom are totally unaffected), 8 sterbai corys. The tank is heavily planted with 0 ammonia, nitrites, or nitrates. The ph is a bit high, around 8, but is that enough to do all of this? (I’m trying to address this by transitioning to a blackwater tank, however the issue started months before I began this.) The tank has been running for years. When I got my first 6 corys, all was good aside from one who died after a few days who I assume was dodgy from the get go. After about a month, one started having some swim bladder problems and it slowly spread through the majority of the group, though the two biggest never had any issues.

This ‘swim bladder problem’ consists of them being unable to stay at the bottom without effort, and occasionally having episodes in which they dash around faster than I can even see, smashing into the sides of the tank and the tank lid, hard enough to injure their faces. Despite this, they continue to eat and sometimes all seem fine.

After this initially started, I increased my water change schedule heaps to try to help whatever the issue was but it didn’t seem to help.

I’ve treated with general cure incase it was anything parasitic which almost seemed to briefly help but in the end didn’t. 

My next move was something I absolutely regret, but I thought perhaps the issues were stress induced as the two bigger,  unaffected corys endlessly harassed the smaller guys, so I got 3 more, bringing the group to 8, with the aim of dispersing any drama/mating aggression or whatever was going on. However, within days, the new guys began to show the same symptoms.

I’m not thinking maybe bacterial? Is it worth hitting up a vet for some antibiotics for my guys?

Please let me know any ideas! I’m desperate to help my guys be happy and healthy again. 

[Caryl]

Sorry there have been no answers to your problem.

I would assume something bacterial to affect all the fish, not just one species. Never add any medications to an aquarium unless you are 100% sure you know what it is you're treating.

Have they had a change in food, either type or brand? I had one brand (a long time ago now so can't tell you what it was) that was fine when the fish ate it but it swelled a lot more inside the fishes than the usual brand and it caused major problems. Swapped food and they all settled again.
In case you're wondering, I know it swelled more as I got one of the new pellets and dropped it in water along with the previous brand pellet. The difference in the amount they swelled was astounding.