Hi ,
I guess Ca, and Mg does not really affect fishes so much , however it does affect alkalnity which in turn affect your pH which is a imporatnt parameter for fishes.
So u have to keep your Ca and Mg at reasonable range.
i would advice u to go for a small skimmer like the prizm as it really help in the long run . Po4 will cause algae to bloom which affect the aestatics of the whole tank, do keep it to minimum.
cheers