Hi ,
WELCOME TO MAAST.
1. Discard the first 5 gallons or so , then use a TDS meter to test the output water. You want it to be around 0 ppm.
2. No, add tab water will defeat the purpose of RO/DI.
3 You need to test the main tank for calcium first (recommend range 380-420 ppm) and then you can match your RO/DI water before add it to the main tank.If you just have a fish only tank then i would not worry too much for calcium.
Reefing is just like cooking, all the ingredients have to be just right , except you don't have to bring the water to a BOIL...
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