water readings advice

hi all i use collected rainwater and im not sure if there is something wrong here but the readings are
kh3
gh7
ph 6.5
I read that for best plant growth that kh should be above 5 i am using co2 so why is my kh so low?

Rain water by its nature is low in dissolved minerals (TDS) which would account for your low Kh readings.
You can mineralise it, as you would with RO water.

ive a similar problem with my tap water, its super soft with kh reading of 1, im using coral sand and gravel to add calcium in order to have good buffering and stable ph. i did read online that you need 1kilo of coral sand to every 40l water to maintain a kh of 6d.

Same water here too. You can also use crushed oyster shells to raise the kH