Yesterday I noticed a previously active and precocious white+yellow
Oranda was feeling punk. I measured some basic chemistry and was
astounded to see +0.5 nitrites and 80ppm nitrates, but ammonia
still zeroish. This was 6 days since my last PWC (according to
the log), and the first trouble I've had in this tank since it
first cycled up over 18 months ago.

A quick 90% PWC perked everyone up in almost no time. There was some
damage done -- the Oranda's dorsal fin shows some red streaking --
but I think the situation is under control now.
I keep a Duetto-50 cycled up and running just for such emergencies,
and moved it into the tank with the PWC. It's too small for a
slightly overloaded 20G, but it's better than nothing. (Slightly
means 3 fish, 2" max.)

The big mystery is - What happened??
I find it curious that at least some bio-filtration survived
whatever calamity struck. Note that ammonia reading was zero,
and nitrates were _way_ elevated. The nitrIte did the damage.

Some possible factors -
- as of about 3 weeks ago, my water district switched to chloramine;
I was aware of this and switched from Novaqua to Amquel, at least
that was the plan. Did I have a blond-moment and forget it when
doing PWCs this month? Did chloramine kill some biobugs?
- The white Oranda had just recently laid a monster load of eggs over
a 24 hour period; all but a half dozen well hidden ones were
then consumed. She went punkish about 48 hours after the egg were
laid. Did this extra "food" contribute more waste than the system
could handle?

Any and all theories (except those based on notions of Karma or
feng shui) are welcomed and solicited.

Gunther, who's off to visit the dentist now.