Kodiak wrote:
90gal, aquaclear 500, cycled, 1 yr running with
small airstone with a small airpump.
snip tank stock
I went on vacation, let someone take care of fish, when I came
back there were 3 or 4 julies lying dead on the bottom. Some
had been partially decomposed and were wrapped around the
filter intake, but filter was still flowing.
snip cleanup
PH 7.8
Ammo 0.1ppm
NO2 0.1ppm
NO3 5ppm
GH 80ppm
KH 50ppm
temp 77

These are not good numbers. You have both an ammonia and nitrite spike.
Your tank is cycling again. The excess ammonia is likely caused by the
dead fish having been left in the tank. Since you were on vacation at the
start of this, you have no idea how high the ammonia spike went
originally. That and/or the subsequent nitrite spike is likely what is
killing off the remaining fish. Various species have different tolerances
to the levels of ammonia and nitrite and fry typically are more
suspectible than adults. The best thing you can do right now is water
changes and filter maintenance (clean ONLY in dechlorinated tap water or a
bucket of tank water) to make sure there's nothing caught up in the filter
that's decomposing. Also, make sure you've found all the dead. Have you
taken apart the rockwork to find any other possibly hidden bodies? You can
also add a couple tablespoons of rock salt to help counteract the nitrite
poisoning. Getting an ammonia locking product for the ammonia spike is
also advised.

In the future, do not use someone to fishsit who is squeamish about
removing dead fish promptly. In fact, if at all possible, do not use
someone who has never had a fish tank. Find either someone in the local
aquarium society that you trust or hire a professional pet sitter who is
experienced with fish tanks (even a marine tank experienced professional
pet sitter is preferred to someone who knows nothing about aquariums).
These sorts of people will remove dead fish promptly and make sure all
filters are functioning properly. They'll also perhaps care enough to do a
head count to notice that fish are dying instead of just dumping food into
the tank.

I had an experience very similar to yours when I let my roommate petsit
while I was at a conference. She left the AC off during a 100F+ heat wave,
so my theory is that the large fish started dying off from low oxygen
levels. She left the dead fish in the tank and kept dumping the food for
them in so that when I came back 3 days later, everything was either dead
or dying from respiratory distress (gill damage from ammonia plus low
oxygen levels is pretty much a death sentence). Ammonia levels were off
the chart on the test kit. The only fish to survive my roommate's "pet
sitting" was Mr. Betta in his seperate betta bowl. She also didn't take
care of the cats in the apartment. So instead of spending $15 a day on a
professional for a few days, I lost over $200 worth of fish stock and came
back to extremely stinky cat boxes and multiple cats suffering heat
exhaustion. Now I always think back to that and hire a professional.