You absolutely need to change that res every 10-14 days... I do it every 7, because I like a steady weekly schedule.... It is not flushing.. that is done end of the grow... you are ''leeching'' nutrients and salts from the plants... When you wanna do it.. say every 14 days... turn ur pump off... wait for the water to drain... dump the water... put in a lil clean water... swish out ne crap... dump that... put clean ph water in.. run it for ~ a day... then new water w/ new nutes..
If you have a ppm/tds/ec meter definately top up your nutes.. usually you dont want nething over 2000ppm for most nute mixes... Remember the plants absorb different amounts of each nutrient. So do your research on what the plants use more of one thing then others.
Ph should be checked every day if possible... every 2 days at the latest... it doesn't take long for the solution to change, especially when the girls are flowering. A couple gallons doesn't go too far.
The reason you NEED to do this is: Hydroponic Nutrients start breaking down the second you mix them together... ALWAYS put the water in first.. then nutes/ph after for that same reason.
Wanna see something gross? (do this in another place then ur growroom) Take a bowl of water.. put nutes in it.. and let it sit for a few weeks... u'll get a nice white broken up chedder cheese kind of effect... that is your nutes breaking down.
Hydro is easy to do.. you'll love the drip system.. I haven't gotten any girls in mine yet.. i'm awaiting more roots... but I still like to see it in action... normally aero allows no water to really be seen...
Have you seen what I built on the weekend?
New setup, new design. I'm at the same steps as you.