how often change water in a recycling system

ha.. "light exposure*"

This will be influenced by a few factors. temperatures -- higher = change it more often. Light exposture -- more = change it more often How quickly you have to top-off and how long it sits at lower volumes etc... The greater proportion of 'fresh' added water+ferts the longer it'll take to get nasty. I don't have a recycling system but i do have a 55gallon rez. I noticed long ago not to leave it at lower volumes for any extended period of time. Exacerbated by warmer temps as the powerhead used to agitate/oxygenate will warm a smaller volume a bit more than normal but also because the microbes/liter amps up that much faster with greater surface to volume ratio. I use mine in bunches, yours is depleted in a more spread out way, even if the same amount drunk per increment of time on average. So i'd top off at 50-75%. If you want to learn how far you can push it, or compare procedures to see which last longer between necessary full-refill/cleaning, Just go until you notice any sort of biofilm or hint of a new smell. It's usually pretty obvious when water starts to turn - this won't derail a grow. Now, you know relative to your SOP and environment how long you can go. this helps for planning - like when taking a vacation you'll know if you need to do an early cleaning to avoid issues by the time you get back.

General rule is to clean out and start fresh every 7-10 days. Usually what happens is once you start fresh with a new batch of mixed nutrients, the water that evaporates or is taken up by the plant will get replaced with pure pH balanced water. I know a few people who every month break down parts and peroxide bath them in between to keep everything nice and clean. This isn’t needed but keeps everything nice and clean throughout the grows. However, once you are finished with the grow I would deep clean the whole system before starting again. Hope this helps.