almost all the fish that you have are soft water and sensitive fish. but don't be discouraged, i and a lot of people keep them in alkaline water.
angels at that size are very prone to deaths unless well taken care of. even then deaths do occur.
neons have been weakened by too much inbreeding and i still find one missing occasionally in a well-established planted tank. some tanks just are not made for neons *sniff*
GBR's have an entire thread (a huge one too) dedicated to them.
rasboras and the black neons are supposed to be much hardier though.
i wouldn't be very surprised if you have angels or neons dying off, despite pristine water conditions. let the angels grow up a little, they should be fine then. meanwhile, carry on with the pwc
's and don't worry too much.
lastly, i wouldn't buy a costly uv
sterilizer without ascertaining the reason behind the deaths. if the fish do have stringy and white poo, its internal parasites. quarantine the non-eating fish and look for other indications...