Hi Cuteusagi. Otos are fragile in three ways, which gives them their justified reputation as fragile fish:
- no regular food source,
- stress
- water quality.
regular eating
If they go without algae or some sort of vegetable matter for as long as a couple of days, this can kill their digestive bacteria which would make them unable to digest even when the fasting is over.
You'd know it's likely if when they died they belly was quite sunken in and if they didn't constantly have a poop hanging off them.
water quality
I wonder about the 0 0 0 test results. I wonder if your kit could be inaccurate? This is all in the spirit of finding out what's happened, you know, not having a dig or anything. I know how frustrating it is to loose fish and not quite know why.
stress
Were there a lot of heavily planted areas in your tank? They tend to do better when they have densely bushed areas to retreat in.
Were they comfortable swimming around the tank or constantly hiding?
How big a school of otos did you have? They're really stressy if kept with less than 5 compadres.
Stress can also be caused by invasive or loud maintenance routines; or too drastic changes betwen the old and the new water (pH, temp, etc.). Could that be it?
If they were turning gradually paler and paler in the end, then it could be that stress is what's happened.