Thanks for the helpful responses! The die off seems to have abated
(found a dead clown yesterday, but I think he'd been dead a while). In
general, the fins are unclamped, and the fish seem perkier.

I'll try the greater volume water changes. I'd held off on heavier
changes after having read something persuasive in one of the forums
about a problem it could cause ... don't remember for sure, but maybe
something about "osmotic pressure" or some such?

The whole water chem thing is pretty much a cypher to me. This is a
resurgence of a childhood hobby from 40 years ago for me, and back then
all we tested was temperature & pH, and you NEVER changed water, just
replaced evaporation, so I'm pretty fuzzy on how the whole ammonia
cycle thing, etc., works.

All my tanks tend to run 40 to 80 nitrates, w/ pH 6.8 or lower & KH
around 80. No ammonia detected. I understand the nitrates are higher
than desireable, and the pH & KH combination is weird. However, other
than this recent problem, the tanks seem to have done well for over a
year (live bearers breeding like crazy, zebras & Angels laying eggs,
everyone generally perky), and I'm loathe to start adding chemical
cures when things seem well enough, so have kind of left it alone. Not
using CO2.

I did lose a lovely little Rachovi killifish in my 110 gallon tank this
week, but I understand those are "annual" species with a life span of
about a year, which he'd pretty much used up, so I don't think it's
related - rest of the tank is doing fine.

I test with those 5in1 test strips plus an ammonia suction cup
detector, so perhaps my testing is too primitive for really
sophisticated readings?

I'd asked the family about possible accidental additions, but no one
remembered anything. It's certainly possible -- my basement fish room
is also the hangout & band practice room for three teenaged sons &
numerous friends, most of whom enjoy watching the tanks. Maybe an
unreported spill or some sort?

The thing that seemed odd to me was the snail die off preceding the
fish die off by a week or so.

Thanks again -- Jim