I buy my r/o di water from my lfs for my SW nano tank. Costs me $1.29/gallon premixed, $1 a gallon pure.
I do not do this for my freshwater setups. As a total of 1050 gallons(105 gallons just for a weekly 10%) of water between them would be cost prohibitive.
Just an example as there are options out there.
Theres plenty of videos on youtube for ro/di setups. How to set up, what to buy, what to add, etc.
Do the tests first. Do the nitrate test on the tap. Then for further tests Lots of companies will actually offer water tests for free. You contact them, get a test vial, mail it to them, wait and get the results. They offer these thinking you will buy their filtration systems, but most do not require a purchase. Near me Home Depot does this. Petsmart and petco also offer tests like these if a customer.