Where is your water source? A spring? a river? pond? if its from the community it can be a number of fluctuation problems. If you know the source, then look around it... Like what trees are around it? Often times spring water can vary do to the trees around it and the health of those trees and what those trees are having to filter around it. Also im sure your in a place where there are a lot of growers? Either way do you know what your neighbors are putting in their water? These types of variables are what is most likely your problem. All can be easily solved...
Check it, I myself have had many of water trial and errors being in the mountains hahaha. So I may be able to help.
If you have the tools, and if you have the acres.. Dig a water capturing pond that you use specifically for your indoor. Put trees around it on each axis as to filter all potential runoff water going into it and the trees will keep the water stable. Put a bubbler, fish and some food grade pvc in it (fish like pipes and stuff like that, it calms them down, seriously) then youll get great stable healthy water, that is also different from your neighbors making the opportunity for better price points :D
IF you dont have the room for the stuff outside, simply buying a very large res can accomplish the stability you seek. A good rule of thumb I go by is to have a res twice the size of your full capacity of the full volume of all your containers. Dont bubble that res.... circulate it... put a small pump with a small hose that circulates the water juuuuuust above the res water line. Then either use a very small amount of the compost tea/earth tea in it. do not put a lot cause it will start to raise. a little will only fluctuate a few points if at all which shouldnt be a problem with you feeding your plants and then filling the big res back up with more fresh water. You could even add goldfish and basically forget your nutes.... let me know what your situation is man... hope this helped