PH keeps rising

[millstream]

Hello. I recently built my own greenhouse and am sticking my toe into some hydroponics. I currently have a dozen strawberry plants and a dozen cukes and tomatoes growing. The plant are doing fine but I can not keep my ph down for the life of me. I have been running this system for about 3 weeks and every day I correct the ph to around 5.8 and every 12 hours it is back to 6.8 I am running a mixed drip/NFT (as you can see in the pics), Fox Farm Nutes, air stone and a pump. I am starting seeds in rockwool and then into hydroton. I ph adjusted the rockwool before I germinated the seeds also. I have read where people set their ph and it pretty much stays there 'till then next water change. Mine stays there for about 6 hours.... I am using spring water with a ph of 7. This isn't a big deal but I am just curious as to why my ph keeps climbing. Thanks-Scott