Ok, now onto the cory's
Corydoras should love MWRA water - Soft with a lower PH is ideal for most species. However, as i described above, problems could arise from large ph changes that are inherent in unbuffered MWRA fish tank water changes. And yes, it is possible that an initial change from trop isle conditions to your conditions could have been stressful.
However, the fish lived for nearly a week before they died - and deaths due to PH shock would most likely have occurred in the first day or so after bringing the corys home. And furthermore - we at the fish factory only very very rarely loose fish to one time water chemistry adjustments.
I think it most likely that the cory's developed an illness from the stresses of shipping. When bringing in wild corys we sometimes loose batches to internal bacterial infections - red patches on their sides - gasping - listless behavior. I feel that this is a result of shipments coming in with highly ammoniated - low oxygen water, and not a result of any PH of Hardness matter.
In the future be very careful to inspect all fish in a batch... make sure they are active, eating, and if there are any dead or dying ones, try and figure out the cause. Also, it is always good to note how long a fish has been in a store - new arrivals often haven't had time yet to show signs of shipment stress.
And of course - you should get your fish from us, so we can make sure you get the healthiest stock possible
Goodluck
-Sam