Help! Tank die-off.

My 55 gallon live bearer grow-out tank is having a serious fish die
off. Prior to that there was a ramshorn snail die-off, and I was
wondering if that might be a cause of the fish deaths, or if anyone can
make any other suggestions.

About the tank:
* Contains juvenile to adult guppies, mollies & swordtailss. Was a bit
overcrowed with around 60 fish until a month ago when I sold maybe 30
to a LFS. Of the remaining 30, maybe 10 have been found dead the last
two days, mostly guppies, but also some swords & mollies. Some fish
from this tank moved to other tanks a month ago are doing fine. All
borm & raised in my tanks. No purchased fish, plants, ornaments or
equipment have been added for at least 6 months or so.
* Penguin dual bio-wheel filter. Tank has run with no problems for a
year. Bio-wheels are well seasoned.
* Stats: Temp. 76 -- Nitrates 80 -- Nitrites 0 -- Hardness GH 150 --
KH 80 -- pH 6.8. This is roughly the same as it has been, and roughly
the same as my 110 gal & three 20 gal. tanks. No problems in any of
the other tanks.
* Fairly heavily planted; gravel bottom.
* Fed flake & frozen food twice a day. 20% water change via python
every 1 to two weeks. (Same routine for all tanks.)

Snail situation:
This tank had been a bit overrun by unwanted snails. I'd squish
the ones I could easily reach, which the fish ate enthusiastically,
been doing that for months. About 2 months ago, added a couple clown
loaches from one of my other tanks who seemed to thin them down pretty
good.
About 2 weeks ago I noticed that a few snails were floating, and
squished them like usual. This continued, but I started to notice that
occassionally the squishees would have a slightly bad smell. Assumed
that maybe the clowns had killed them but not consumed them & the some
of the floaters were a bit ripe.

Fish die-off:
Left town for a couple days & my wife fed tank while I was gone (a
standard procedure for us, she knows how to feed). When I returned I
found several dead fish & many more dead snails than usual. (Rather
than squishing them, of course I removed them at this point). Tank was
a bit more murky than usual. Other fish were sluggish w/ folded
dorsals. The past two days, I've lost a couple more each day. No
other signs of common diseases.

What I've done so far:
* 40% water change.
* Add charcoal to filter.
* Run aux. diatom filter 10 hours.

Is it possible that the snail die-off added contaminants that
contributed to the fish die-off? Did I release toxins by squishing em?

I'm assuming this is some sort of water quality / environment problem,
but what?
What else might I try to help?

Any advice most appreciated. -- Jim