PH rise in the reservoir

Running drain to waste system. The water is RO with re-mineral filter. After adding silica, calmag, ghe flora coco nutrients, the ph drops to 6.5-7.0, adding ph drops it to 5.8. There is a water pump in reservoir, it stays on permanently. The ph goes back to 6.5 after 3-4 hours. What could be the reason? I’ve tried to add tap water, didn’t work. Tried to use only calmag and basic nutrients, still no luck. Any ideas?