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hypoxia in a planted tank?

One of my goldfish has been ill--swimming oddly "off kilter", eating
poorly, fins clamped, one of them red at the base, and without other
external signs of infection or infestation--while his companion is her
usual bouncy piggish self.  I posted my concerns on another forum, and
someone suggested that my fish might be suffering from hypoxia.  Does this
sound reasonable to you more experienced folks on this list?  The fish got
worse the day after a routine every-2-week 50% water change with an
unusually aggressive cleaning.  And could a deep-cleaning of the tank
(including scrubbing at lots of algae and cleaning out the gunk in the
filter trays) have stirred up the bacteria enough to make things worse (I
say worse, rather than started it, because the fish started looking ill
before the water change)?

Tank parameters:
2 6-inch goldfish
29G tank with Eclipse hood, biowheel, nylon mesh in place of purchased
carbon-filled filter unit
Tap water conditioned with declorinator/dechloraminator
pH 7.2, nitrates and ammonia zero, kH and gH both mid-range on my test
strips (don't have the numbers in front of me, but they were smack in
middle of the "good" zone)
T82 degrees (since summer hit--was 74 in the fall/winter/spring, but even
with air conditioner to 74-76 degrees, lights heat it up)
Heavily planted (and moderately algaed!) but mostly with slow growing
plants (fish eat all the fast growers...)

I was injecting CO2 but the tank ran out last week and I haven't yet been
able to get a replacement (those places aren't open reasonable hours for
those of us who work too many hours at the hospital)....and I'm using
needle valves so I hope that there wasn't an end-of-tank CO2 dump (plus
other fish in other tanks run from same setup are unaffected).

I will definitely drop the water level to increase surface agitation
tonight, but am not sure whether I should be investing in a pump/airstone
or a fan to cool the tank or just saying uncle and persuading my friend
with a pond that she really needs two more fat spoiled goldfish......

--diane b, in st. louis where it's hot but not especially humid today