Tank is slowly dying off
My male betta Malcolm X died today and I need help figuring out what caused his death.
I started my first planted tank in February and, after a few months of cycling, added a betta and 11 pygmy corydoras.
The tank is 70L but is filled to around 60L. I test the water every two weeks because the bio-load is so low that I've never had any significant issues with my water parameters. They usually sit around:
- Temperature 24-26C
- pH: 6.4–6.6
- Ammonia: 0 ppm
- Nitrite: 0 ppm
- Nitrate: 5–10 ppm
- GH: 3–5 dGH
- KH: 0 dKH (it's a planted tank with aquasoil)
For the past three weeks I've noticed a gradual change in my betta's behaviour. He stopped being interested in his usual pellets and would instead just swim around the tank aimlessly. He also seemed to be sleeping more and spending more time near the surface. Throughout this time I never noticed any visible changes to his body, but over the last few days his fins became clamped.
I'm a first-time fish keeper and today I wondered whether low oxygen from insufficient surface agitation could have been the cause. I did my usual water change, added some new plants, cleaned the filter and increased the flow. I was working on the tank for around two hours, although I wasn't constantly disturbing the tank during that time.
When I finished, I noticed he was floating vertically but would occasionally correct himself and swim normally again. Around two hours later I found him dead.
Although I mentioned oxygen as one possible cause, my main concern is that this may instead be a bacterial infection or some other underlying issue.
The reason I'm worried about this is because I've had ongoing problems with my pygmy corys as well. I originally ordered 11 online, but when the box arrived there was nothing on the outside indicating that it contained live fish. Although they were packed well, they seemed very stressed from shipping.
Two died within the first week. Then, over the following two months, the others continued dying off randomly. Two of those deaths I attributed to swim bladder disease because they lost the ability to stay upright, but the rest died without any obvious symptoms.
Of the five I have remaining, one is missing an eye but otherwise appears healthy. Another two have become noticeably rounder, which has me worried they may also have swim bladder issues.
What I'm struggling to understand is why my fish keep dying despite the tank appearing to have suitable conditions for both species. The tank is planted, the water parameters are consistently stable, they're kept at the correct temperature and they're fed an appropriate diet.
The only other inhabitants are bladder snails that hitchhiked in on some plants I bought. They don't seem to cause any issues apart from occasionally eating the corys' food.
Has anyone experienced something similar? Does this sound like a bacterial or parasitic infection, a problem introduced with the original shipment of corys or could there be something I'm missing about my tank despite the stable water parameters? Is there anything I should do before deciding whether to keep or rehome my remaining fish?
I know five pygmy corys isn't an ideal group size, but after everything that's happened I don't want to introduce any more fish until I understand what's going on. Otherwise, I'm considering rehoming the remaining fish.
I'll attach photos of the tank, other tankmates and my betta after he passed away for reference.
