Caper, that is such a shame!
I have no idea what is wrong but since two fish have died it may be something that they brought in with them.
Your ph shouldn't be any problem I wouldn't think. And you are right about the water changes, as long as the parameters are fine a weekly water change shouldn't be a problem. It is only if the tank is cycling or, once it has cycled, the nitrAtes get too high before the next change that the water change size or frequency needs to be increased.
Was Squirt one of the new ones? I hope everyone else hangs in there!
Best wishes
MCT