Another feeding question - Purple Firefish

I've had a group of three Purple Firefish for quite a while now, at
least a year. Recently, one started losing weight and finally expired.
Now, I notice one of the others is looking a little light, and I'm
worrying. The other fish in the tank are eating and keeping their
weight on. I feed everybody a variety of stuff, from flake through
"human" fishy stuff. The firefish eat whatever they can get in their
mouths, which has been fine, since I try to be sure at least part of
the feed is very tiny.

I wonder if they just have lived their lifespan, or if something else
is going on. None of the other fish seem affected. The water
parameters seem good, with the exception that the pH drifts down and
has to be buffered up with our water changes.

Does anyone else have experience with firefish, and can suggest some
course of action or special food?

=^..^=
.... the problem with people these days is that
they've forgotten we're really just animals.