Reading through this thread, there are a couple things to mention.
First, the issue of the water pipes repair/replacement causing the water to be unsafe to drink. I've no idea what this might be. When we used copper pipes several years ago, copper was a serious issue, as the "safe" level of copper for humans will kill any fish put into the water. Today they don't use copper (though older buildings may well still have it, and it may need repairs), but even so I suppose other toxic substances can be relevant. I can only point out the possible issue, I've no idea what it may be or how to deal with it (other than not using the water obviously).
Second, water circulation. Air stones do not cut it, period. Fish that need a current--and panda cories do because they occur in small mountain streams in Peru and are designed for these, also cooler temperatures for this reason--will not benefit from airstones. The filter must produce a current across the tank or down one end wall, depending upon the filter mechanism. It is unlikely low current would kill the pandas like you describe, but as the issue of current was raised, you might as well understand it.
Third, there is always the possibility of "bad" fish. Mass-produced fish as opposed to wild caught imports are more likely to be infected with "x". I absolutely will not buy fish from chain stores; the fact that most of the fish I used to get from such stores ended up dying or causing problems over a 2-year period convinced me they are not worth the risk. Disease spreads easily in commercial fish suppliers.