i'm confused. is your ph going down in your dwc or in your coco grow?

some ideas as to why your ph may be dropping but nothing definitive:

1) you're over feeding your plants. if there is too much food in your res, the plants will drink a greater proportion of water to food ratio, thus increasing your ph as there will be a higher concentration of food to water present in your solution.

2) salt build up. as you know, the salt buildup (crusty white stuff) can greatly increase your ph levels. this can occur if you are using a coco coir that possessed salt buildup (the coconut husks come from tropical places that are sometimes left on the shores for extended period of time and absorb sea salt). you may need to flush your coir next time to avoid this problem.

3) if you're room temps are too high the water will evaporate quicker and increase the concentration of food to your plants. generally, when the temps are higher you may need to add a water feed more often than when they are lower. eg. food, water, food instead of food, food, water.