The ammonia had been stable like that for 2-3 weeks there is still nitrates being producedWhy is your ammonia present? Are you not cycled?
Albino, and some other cory can be invincible in good conditions, and some are known to survive harsh conditions.
Just because I said that doesn't mean nothing can kill them, it's just how they are sometimes.
My albinos went through a spike of nitrites when my filter stopped overnight, and I didn't realize it.
The tank is a 30 gallon tall octagon 2 mollies 2 guppies 6 danios. Few fry 1 Cory and 3 shrimpIn order to be cycled, you should be seeing 0 ammonia, 0 nitrite, and some nitrate. Just because you're seeing nitrate doesn't mean you're cycled.
Cories, IMO, tend to be pretty delicate when it comes to water parameters. Any ammonia or nitrite in the water seems to stress them pretty quickly, when compared to other popular fish. With .25 PPM ammonia for 2-3 weeks, it probably slowly succumbed to ammonia poisoning. I'd get that ammonia down ASAP.
If you're having a consistent ammonia reading, it sounds like its one of a few things:
1). Your tank never got to finish it's cycle, and is now doing so quite slowly.
2). You don't have enough filter media for BB to grow on.
3). You're overstocked.
4). Your water source (tap, or well) has ammonia in it.
Can we get your tank size and your stocking? As well as your filtering situation and what ever media you have in it?
I'd go ahead and run an ammonia test on your water source as well.