dedraic

I woke up this morning to find one of the two Otto's I put in my planted 10g was lying on the bottom of the tank. I nudged him with my net, and realized he was dead. I can't seem to figure out why though. My pH was a bit lower than normal this morning, which was expected because I added a home made CO2 setup yesterday(I suspect this may be the cause somehow, I accidentally added far too much yeast and didn't realize it until this morning).

Last night he was also edging up the side of the glass, and remaining motionless for greatly extended periods just below the surface. I know there's enough algae in there for them to eat, which is why I got them.

My water parameters are:

Ammonia- 0ppm
Nitrates-10ppm
Nitrites-0ppm
pH-6.5

And the tank's occupants are now:

1 Otto Cat
4 Neon Tetra
2 African Dwarf Frogs

All of which are perfectly fine this morning. Everyone is eating, and I'm not seeing any peculiar behavior(this is my desk tank, so I'm watching it pretty much all day). I've had everything but the frogs for about two weeks, the frogs being a mistake I bought when I first started keeping about four months ago, but very cute mistakes nonetheless.

So, can anyone help me out on what killed him? There was no mark on him, I looked him over with a magnifying glass. His underbelly seemed abnormally yellow as well. I really like the little fellows, so I'd also like to know if it's possible to tell if it's safe to add one back this coming weekend.

TFA101

I've heard about ottos dying mysteriously with no physical symptoms or behavioral issues at all. Some of my friends have had this experience, I think it was just a stress moment on the high end of the scale. On the other hand, he could've been killed by one of your frogs, which I highly doubt. But I think it was just an old age thing. I dont know what it could've been.