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I dunno what I'm doing wrong, but the 2 Cory sterbai I bought this weekend both passed away last night.

I don't want to give up on this species (I have 2 and love them) but that is a total of 4 that I've gotten from you that have died a day after looking fine and healthy. I don't get it and I'm frutstrated.

sheesh boy...
I don't know
mine are all still fine

a couple things about Corys in general

Corys and most catfish need to be able to get a bubble of air occasionally
so don't fill the tank 100% to the top
(an inch or so shallow should be good enough)

They are of course bottom feeders
so make sure some food at least
gets to the bottom occasionally

I sometimes over - caution people about over-feeding

we have things like shrimp pellets and tetra color bits that sink
they also like things like frozen brine once in a while

pH is not critical : anywhere from 6.0 - 8.0 should be fine for Corys...
the stability of the pH is more important than the actual number
...it's a good idea to test your tap water pH AND your tank water pH...
you want to within say a half a point...
so if your tank runs at pH 7.0 usualy and your tap water runs at pH 7.5...
you do not need to add any chemicals except chlorine remover such as Aqutan or Novaqua

your particular Cory is native to Central Brazil
(your exact Sterbai happen to be captive bred)
so their ideal temperature should be 77 degrees F.
...safe range would be 68 - 83 degrees F.

(I think I already put this same info in another post)

no other particulars I can think of...
except Corys don't like to live alone...
I would house a minimum of 3 together...
only other thought is a few of the SOUTHERN Brazil corys
can't stand the water above 84 degrees F.

Yeah, i'm stumped. I use RO water, no tap water. 78 degrees. 6.8-7.0 Ph.

I have 2 that are fine, every time I had another 1-2, those ones die within a week.

They get food, use shrimp pellets and/or freeze dried tablets. Frozen brine once a week (one cube/tank).

1" at top for air.

Dang. I might try again if you still have them in a week. Maybe a nitrate/nitrite problem.

I'm changing 7.5 gallons on average a week in the 35 gallon tank.

"I use RO water, no tap water. 78 degrees. 6.8-7.0 Ph. "

this is highly suspect

Have you checked your pH ????
with the lights on for 5 hours
AND
also with the lights OFF for 5 hours

pure (R.O.) water is always pH 7.0 exact
UNTIL
you do ANYTHING to it
(by anything I include having a quarter ounce fish breathe in it)
which makes Carbon dioxide
which makes pure water pH FALL LIKE THE STOCK MARKET

what I'm suggesting is that your water has ZERO calcium dissolved in it,
so your pH is dropping catastrophically LOW ...at night in particular.

I strongly suggest you use at LEAST 10% tap water
OR
use a product such as "RO RIGHT"
to add back a minimum of hardness

it's possible that your original Corys are used to the ZERO hardness
and when you get new fish it's pot luck if they get used to it or not

I haven't checked the Ph w/ the lights out. I always thought the abundant plants moderated the Ph a bit (soaking up the CO2), so I haven't given much thought to it being 7.0. It is definitely not an acidic tank.

I'll start adding some tap water, after aging it.