If ur calcium level matches ur carbonate level in balanced ion-proportion. Then lower the KH then slower the growth as ur sps consumed less calcium-carbonate for growth. And if u matches lower KH with low nutrients then sps tissues will disperse the brown zooxanellate resulting in brighter colors. If u go beyond that and achieved ulns, colors could look pastel in some or most sps. This is sps natural abitlty to photosynthesize with the symbiotic zooxanellate algae living in its tissues.