I don't think the above guy is talking about DWC since bubblers don't use external reservoirs.
Here's what I do with a similar setup. Keep in mind I'm a newb and still learning but so far so good with excellent results thusfar in the last 6 weeks. I use GH nutes and the new lucas formula.
I use four 12.5 gallon rubbermaid tubs, each housing 3 plants for a total of 12. I have each plant in a 5" net cup, and a 5" round airstone under every plant. To make the nutrient solution, I put in 9 gallons of wal-mart distilled water in the tub. I then add 45 mL (5 mL/gal) of Micro, stir, then add 90 mL (10 mL/gal) of Bloom, stir, then add 9 mL of Superthrive, stir, 75 mL of Hygrozyme, stir, a pinch of roots oreganum XL, stir, then 6.5 mL of pH up, stir, and my pH and ppm come out to 5.6 and 610. My pH is constantly becoming more basic, and about once a day the pH goes over 6.0 and I'll adjust it back down to 5.5.
I started off thinking that weekly changes were the way to go but after researching I decided that it's not necessary to change the water that frequently (at least not for the early veg stage I'm in) and I now completely dump out the nute solution every 14 days and start over.
I top off with distilled water only, and adjust the pH as needed when I do so. I don't add nutes in between the 14 days unless total ppm goes below 500 in which case I usually toss in a few mL of Superthrive. When I didn't have roots coming through the net cups, I added a gallon of water every time the water level dropped below the bottom of the net cups. Now that there's lots of dangling roots I plan on letting them drink up and will add 2 gal of water once they've drank 2 gal to encourage further root growth.
Hope that helps.