what causes ph to drop drastically over night?

hi, everyone. I run a rdwc setup and when I mix up a fresh batch of nutes and do a res change my ph stays stable at 5.8-6.2. my question is after about four days the ph starts to drop a little bit till it slowly gets to like 5.6 then overnight it drops below 5.0. I have no signs of root rot or anything the plants seem very healthy also. I was told by a friend of mine that the plants are eating all of the nitrogen out of the bloom nutes and the solution becomes unstable. any thoughts.

i've looked around because i had the same question. and like your friends said, as your plant is using differnt nutrients it changes the ph the same way as adding nutrients. if i'm wrong someone correct me.

yeah I tend to agree with the whole nute thing. I was just wondering if anyone new the reasons for sure. its weird though my total system holds 12g of water and everyday the plants drink 3-4g I have tried the water only top off thing and the add back whats neccessary to maintain your ideal nute ppm. the first couple of days on fresh nutes they drink an eqaul amount of nutes and water. like you know it starts out at 1250 and the next day its 3-4g low but still 1250. but after a couple days they dont use as many nutes and the ppm rises. even with topping off with water it will bring it down initially but the next day the ppm is way high and ph starts to fall drastically.