I see you are not getting the livestock all from the same store? I found this issue with one store where it seemed everything I would bring home would be dead within a couple weeks. Needless to say I avoid buying livestock there
You tank is pretty well established, 9 months as the 90 I gather and a couple years as the 77G so even with the transfer you would have a pretty stable mature system so unless you started doing something drastically different your paramaters should not fluctuate too much.
You KH is a bit low, but Ca and PH are in line so if you mess with the KH too juch you might end up throwing the other measurements out of whack.. you could look at buffering but try slowly and montior monitor monitor. I would double check the ammonia, I find those test kits to be kind of useless they are brand new, after a couple months on the shelves they always seem to give a false positive.
Have you meaured O2 at all.. what is the flow like in your tank, with a sump and skimmer in theory you should be getting decent surface aggitation (did I spell that wrong? lol) so I'm not sure... although you think you would see issues with your existing fish if that was the problem.
hmmm.. I hate losing fish.. especially when its hard to figure out why?
What are you using for water and salt?