I know this may seem like a dumb question,but I am using regular tap water, ppm from the tap is 459 so would the 300 ppm be on top of the 459 making it 759?
That's seriously high. I think that's higher than what's allowable for drinking water but not positive. You do add the nute number you want to your water ppm to get the grand total. I wouldn't worry about using CalMag but I would add extra Mg by way of Epsom Salts to balance all the extra Ca sure to be in water that hard. It's in a form not easily used by the plant but can still lock out Mg when it's too high.
With tap water a complete change should be done once you have added back as much water as your system holds to prevent salts buildup from hard water. Weekly changes aren't needed when the plants are small to medium in size and I've done many DWC grows without ever changing the nutes but I use RO water and monitor the ppm that is all nutes.
Keep track of how much water you use to top up so you know when you replaced the total your pail holds. With a small plant you might be able to go a month but once it gets big you might need to change every week or less. I use a drill pump to drain tubs when I have a DWC ScroG going. I always cut a smaller hole for adding water, nutes etc and can stick a hose in there and drain 30L in about 4 min. Piece of window screen over the end of the hose to keep roots out of the pump held on with a SS hose clamp.
Almost all the minerals in your tap water are still in the water as you are adding more high ppm water. By the time you add an equal amount of water you have doubled the ppm from tap water so it's now 459 x 2 = 918ppm. Same thing happens when growing in pots. The minerals build up like the scale in a kettle and often leads to toxic salts buildup that starts turning your leaves all crispy in the middle of flowering. Even in veg if you veg for long times like filling out a screen.
