I read somewhere only organic nutes will create algae, but that could be incorrect so take it as a grain of salt.. For me, I mix up 2L of nuted water each time, and it lasts ms 48 hours (2 days). I water using 1/4 of it twice a day. I am slowly noticing I need to put more than 1/4, so kind of coming onto 1/3 or 1/2 of the 2L each day, now that it's bigger. How much all depends on the size of your container, but how frequent is important for Hempy - she likes to have airated water as often as possible (since there's no bubbler like in DWC).. You want to put fresh water in as often as you realistically can. I would think you could go full NFT or automatic drip type watering but then it's not really 'passive' hydro anymore, but that would give it more frequent airated water. Otherwise a few times a day is great, if you can. Some do it once a day or two, but to me that doesn't seem frequent enough. Oh, I leave my 2L of water in my grow box (and yes the light was on it 24/0 and 18/6) and haven't hard any issues with algae or coloration of the water.. or smell, but I'm using non-organice nutes (Flora Duo) and like I said it only sits at most 48 hours.