From a fellow grower:
I’m using a 25gal. Dwc set up for 2 plants. Sense it’s 25gal. And only 2 plants do I need to change the nutrient water every 7 days or can I do a nutrient change longer than 7 days and just watch pH daily? If so how long can I go between nutrient changes?
No longer than 10 days .
If everything is entirely light tight you can get away with over 2 weeks but if you have your tds ppm in the right range you will want to change res within 10 days. Here’s how to judge if the ppm stays constant as water lvl drops this means your plants are consuming nutrients as fast as water if ppm raises you have too high ppm to start. Your ppm should lower everytime you top off so at 10 days if nutrients are correct levels you would be wasting little to no nutrients
I would get a smaller DWC reservoior. In your case; Let me give you a old time DWC tip.
As you know, you need to top off your DWC res’ with fresh PH’d 5.8 water every day or 2.
NOw let’s say that you use 20 gallons in your Res’. The rule on topping off says; " You can top off as much as you like between nutrient solution changes but, never more than the original amount of water), which means you can add up to 20 gallons of fresh PH’d water.
So; You could stretch out your schedule by seeing how long it takes for you to use the extra 20 gallons of fresh water. I would not recommend going longer than 2 weeks give or take a day, before adding totally new solution.