Welcome newbie!
I assume that you probably didn't have enough time to 'cycle' the tanks going by the above, though that is only an assumption? If that is so all you can really do at this stage is water changes every few days as you are now. I assume (also) that you are using tap water and that may be why you have a high Ph, though substrate etc can cause that but is it too high? Shrimp are very sensitive and delicate so they should be acclimated over several hours using some sort of drip system. The last possible problem could be the betta with shrimps, my first betta ate them but my current betta doesn't so that is one of those may/may not work.
A bit more detail will mean we can try and help more, the above details are based on assumptions and 'usual errors' as there isn't really enough detail in your post to do much else?
You appear to be doing quite well if you have just lost 2 shrimp so far! How long have you had the tanks running? What is in each tank (fish, shrimp, substrate, decoration, filter, water source etc etc)?
Simon