Do I really need to do a 100% water change?

[sawdavis]

After I did a 20% water change, I added 12.5 ml per day for five days. It went from 280 to 300 after first dose and has not budged since then. I use Reef Crystals. It says on the canister label that at 1.023 salinity, it provides 405 ppm Ca and 1200 ppm mg. I have been using API test kit for CA and KH and Salifert for the Mg test. I had the local fish store test right after I did and the results for the CA was the same as was the Mg. My KH test results were a little lower than theirs. I would like to go to 420 ppm on the CA and keep KH between 8 and 9. I don't know about the Mg. I never had a problem with that until now.
As for the 100% water change, I have a small RO system and it would take me a long time to do that. I also don't have the storage.

 

[Randy Holmes-Farley]

Use a calculator to determine the dose needed for a rise. there's no real uncertainty in the rise, except perhaps how well you estimate the actual water volume.