Need help: Cories sensitive to water??

[yonsu]

Hi everyone,
I currently have a side job at a LFS, and we experiencing great difficulty keeping our Corydoras aenus (bronze/green & albino) and Corydoras paleatus ("peppered") alive. We receive shipments weekly of 12-18 of each species, and although they appear fine for the first day or two, they start to die off en masse at around the start of the third day. We carry over 200 other species of fish, and these are the only guys that we are having serious problem with. We have tried various acclimation procedures and switching up which tanks they go in, with no luck. This is relatively puzzling, as these guys are supposed to be among the hardiest of bottom feeders.

Our water parameters are as follows:
Water temperature- 80 degrees
Nitrate- 0
Nitrite- 0
Ammonia- 0.25
Hardness (gH)- 150
Alkalinity (Kh)- 60
pH- 7.0

Any ideas? Insight is greatly apprciated!

 

[Lupin]

I see the issue. You have 0.25 ammonia. Your tank is not cycled to me. How come you have zero nitrate? What test kit do you use?

 

[yonsu]

We just use the strip test kits, but they are rotated out quite frequently, so we do not allow them to oxidize much before switching to a new kit. We have also had our water tested professionally.
I understand that ammonia can be hazardous, but the problem is that all of our tanks run off of a central system. Water is constantly being introduced into and taken out of each tank. Professional testing has revealed that the tap water linked to our system contains a massive 6.0 ammonia level, but by the time it is exposed to the fish, it is already reduced significantly and has been ionized into ammonium.
Normally, I would agree that that the ammonia would be the source of the dilemma, but, given the circumstances here, I do not believe they are. However, I cannot figure out what another potential problem could be. Like I said, it is truly puzzling, as we have over 200 species of fish that thrive off of this very same water (even fish known to be otherwise as sensitive, such as cardinal tetras), and yet these are the only guys having problems.
Thanks for your help, and again, any insight is greatly appreciated.