There are 2 reasons for the pH getting down like this. The first is a sick root system. Lift them up to check the roots but you would probably smell it already when with this was the reason.
But it sounds like pathogens in the reservoir, possibly growing in the pump/fittings/tubing or something in the reservoir. If he lets it sit in the reservoir for a day and the pH still starts to go down, that's the issue.
You need to clean the whole system. Just follow these steps to clean the system:
1. Remove just your water reservoir first. Clean thoroughly with dish soap and water.
2.Fill with 2-3 teaspoons of food-grade hydrogen peroxide per gallon of reservoir water. (Be careful with food-grade hydrogen peroxide, it’s much more potent than its drugstore counterpart, but it’s pure.)
3. Run the solution through your system. Run the water in your system for 15 minutes, so the solution can clean all the tubes and the water supply.
4. Empty the water reservoir. Rinse and re-clean the water reservoir.
Disassemble. Take your hydroponics system apart—I’m talking about the water reservoir and tubes, anything water runs through.
5. Hand-wash the parts with hot water and dish soap. I know it’s tempting to use the dishwasher for this part, but it won’t work and the heat can also warp the parts so they don’t fit back together as they should … and then you have no system.
6. Soak in vinegar. If you have a large sink or bucket, soak all the pieces in a solution of 1 part vinegar to 20 parts warm water, and just let soak as long as you want.
7. Replace the reservoir and run the system. Before you put any plants back in, run the water system for five minutes to dilute any hydrogen peroxide still in the lines.
8. Start your system back up, or close it down. At this point, you can start anew, or you can leave it until next year (just don’t leave any water in the reservoir, let it dry out first.)
After this your pH should be stable again. And do this between every grow when you want to prevent these problems.
Happy Growing! 😋
