the number one reason why anyone has a nitrogen def is because there's not enough N in the fert mix. Adding fert to water gets the ppm to raise and the pH to drop. If you're seeing yellowing lower leaves the number one reason why this happens is because of a lack of nitrogen. I think you need to raise your ppm to 1500 and check this a pH more frequently if you can. It sounds like your ppm is too low to begin with and it's probably lowering rather quickly as your pH rises. If this is the case then I think it's your ppm, not pH, that's the issue. When you put that together with yellowing lower leaves, which indicates a need to nitrogen, then I'd say they are eating alot. You can either raise ppm to 1500 and see if you get better stability, or check 1300 more frequently and keep it there the best you can. But this means you have to be available to check the ppm alot. If you can't do this, then I'd raise the ppm to 1500 or so and see what happens. Another issue is the time factor. You can try 1300 or so and run pump for half hour every hour or try more than just once after lightsout. Or mix it up - 1500 with longer run during daytime and only once at night. You have a little more control with the time factor in a reservior system, so try more frequent runs.