Parameters in my tank

[Samispeed]

I have a 12 gallon planted aquarium.
And it's been running for maybe 5-6 weeks can't remember how long exactly.
I had 8 cherry red shrimp and they all died one by one.
So I got testing my water.
Ammonia -0
Nitrite - 0
Nitrate is .25 ppm apparently it's fine.
My ph is 7.4

My gh and kh are sky high!!! Literally off the charts!

Is this the reason why my red cherry shrimp died? Also how do I lower my gh and kh?
Regards
Sami

 

[aqua_chem]

Is your tap water naturally that high?

 

[Samispeed]

Is your tap water naturally that high?

No idea I haven't checked this.
I am in the process of switching to R/O water.
Can you naturally lower kh & gh? Or would I need the aid of buffers?