PH down ?!

could be bacteria... ph-balance it as best you can. some nutes come "ph-buffered" which help reduce ph-drift... maybe, these nutes are not buffered? my pH stays rock solid and i clean my rez every 60 days and plants look fine. Never a full refill during that time, but I do refill 75% at a time back to "full" every 3 days, so it's fairly fresh water on average over time, but still not wise to do like this. Don't try to go 60 days between rez cleanings lol, if that needs to be said. the point is, if pH is drifting like crazy, it's very likely related to the product you are using not being optimal for hydro/aeroponics context - desptie any promises made by marketing material. make sure ph device is callibrated. Try an independent ph test, if possible. Maybe, it's an illusion? 5.5 is about to be a problem if it drifts any lower.. Ca will be locked out or be less available than the concentration fed should be. If you don't see symptoms start to arise below that threshold (not necessarily over night), then it may not actually be happening -- i.e. equipment error, hopfeully.