Re: green spot algae
The two main variables for GSA: low CO2 and low PO4.
So you need to add PO4, like to 2-3ppm and make sure you have good stable CO2 ppm levels.
KH should not effect anything here. Co2 stablility definitely will.
Blend tap with the RO to get about a KH of 2-3 or so, if you want a higher KH etc, that's okay too.
Regards,
Tom Barr