I used to have a 30G SPS dominated reef (decommed). Weekly WC of 10-15% with a good salt should replenish and maintain the parameters. When your SPS are growing well, you will notice that WC can't catch up with the consumed carbonate and calcium. Then it's time to consider dosing with stuff like C-balance etc with a dosing pump. Do not do manual dosing since ur water volume is small and any slight overdosage can be disastrous. Do take note of ur SG swing due to evaporation and how u do topping up.
Xiggie is right, sort out the Nitrate and Phosphate issues first. WC is key!