Hydroponic flush

Better products will resist pH drift better than others... if they hired someone with a proper education and experience to help engineer the products.. Even so, regular reservoir maintenance is necessary to avoid it.. whether you can top it off for 1-2 weeks before a full res drain and refill depends... You do whatever is required for your garden to avoid problems. temperature of the water is a big factor in how fast you get fouled tank.

If it’s just plain water, it shouldn’t change too much. The ph fluctuates as the plant uses up the nutrients and the change in concentration changes the ph.