Biocube Problems... (GH/KH)

[Skate3002]

I just recently lost a bunch of fish after having no problems since it was set up a few months ago. I bought the API freshwater test kit and checked my levels and they came out...
Ammonia - 0
Nitrite - 0
Nitrate - 20
pH - 8.0
Temp - 26 ºC

i was going to use API pH down to get my pH to about 6.5 to 6.8 range (Neon tetras, Glowlight tetras, Harlequin rasboras) but the guy at the lfs said it wont work if my carbonate hardness is out of wack... i picked up the API GH/KH test kit and got...
GH - 11 º 196.9 ppm
KH - 8 º 143.2 ppm

Apparently for my fish i want KH and GH to both be in the 3º - 6º 50-100 ppm range. i have tried reading up on how i can bring these down but i havent found much other than boiling my water. i also read something about peat but cant find any info other then what its used for. any help would be greatly appreciated . thanks

 

[Skate3002]

the stock in the tank are as follows...
1 neon tetra (lost 7)
6 glowlight tetras (3 are albino)
5 harlequin rasboras (lost 1)
1 orange hatchetfish? it was a freebie the guy put in the bag by accident i think it is "chela dadibujori" if i ID'd it right.
5 amano shrimp
1 spotted sailfin pleco (thinking of trading for oto cats if i can solve the pH problem)

i also lost 8 rummynose tetras that seemed healthy right till they died.

2 rotalias
dwarf baby tears

i added the live plants to try and fight my algea and they look much better