300 is on the high side my usually like 150 or so
So have you growth any plants with this water before
weed or even house plants if they did okay you should
be alright. When you mix your nutes you subtract
the 300 from the total on the meter this is your actual
number of ppms of nutes that you are using.
I honestly would suggest you investing in a RO system. They're roughly about hundred dollars or just take a few 5 gallon buckets to a nearby filtration vendor and fill it up. Using tap water will take away much needed nutrients. If your using your tap water which is reading 300 ppm then and your trying to maintain a 1000 ppm level. You would only be allowed to used 700 ppm of nutrients. The RO system will filter out almost 99 percent of tap water. My reading after my RO is around 9 ppm but without it I was getting roughtly in the 400 range. Hope that helps.