Bala Sharks sudden death

Drat, dang, and lots of punctuation. Night before last, I was admiring
our Balas. How they've grown, how healthy they look. Skipped morning
fish feeding while hurrying to get myself to another appointment. Last
evening, one Bala swimming around still looking quite healthy and happy,
though quite alone. Three dead Balas floating. One Bala missing
entirely!!

Surviving fish still seems quite normal. Dead fish, well it's hard to
tell after they're dead (duh) but the gills didn't look damaged, no
obvious damage elsewhere on the fish, no bloating. Just... floating
upside down, dead.

120 gallon tank
2 Emperor 400 (biowheel) filters
no detectable ammonia
no detectable nitrItes
nitrAtes 40 ppm
pH 7.6 and stable
temperature 78F and stable

20% water change weekly; last water change the day before the die-off.

Tankmates: 5 juvenile Bala Sharks (now one)
11 Tiger Barbs (all look and act normal)
1 Liposarcus pardalis, common pl*co (picture of health and asking
"where's the squash?")
3 Striped Raphaels (one found, big and healthy; two not seen but
presumed to be fine; and yes, Raphael cats are like that)

First thought is of course contamination or bad water change. But only
the four Balas were affected! We dechlor water before adding.
Contamination, I don't see how but has the same problem, only these fish
seem to have been affected. Can't find a dissolved oxygen test but with
two filters and the bubbler, shouldn't be a problem; also, no signs of
respiratory distress in any other fish (nor in these fish the night
before!).

ANY ideas? I've tested ammonia two ways and am now taking a sample of
what my tests say is pristine water to the lfs to see if their tests say
any different. Sure would love to know what happened and if it can be
prevented from happening again.

I wouldn't mind knowing where Bala Number Four went, either -- we've
checked around the tank for fish jerky, though there aren't any big
holes for jumpers, and found nothing. However the Raphaels are
protien-loving scavengers and I wouldn't put fish fillet entree beyond
Tess the common plec either. So that may not be a complete mystery.

Wail! Sob! Alas, alack, and WOE! what happened to my beautiful
Balas?!?!?


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Only know that there is no spork.