Ok I am a little confused because you said it goes up when you add fresh water, but then you said it can go from 5.5 to 7.0+ overnight. So are you saying it goes up both when you add water, as well as without adding water? I also forgot to ask how many plants you are growing in this system, as well as how big they are?
You said you have a 45 gallon reservoir you fill about 3/4 of the way. That sounds like about 33-34 gallons. But if I'm not mistaken you are using bato buckets, and each one of those would hold some water as well. I'm still not sure how much water your adding on a regular basis as the plants drink it up (1 gallon a day, 10 gallons a day, 25% etc.). I'm wondering about how much flocculation to expect (depending on water to plant ratio), as well as pH change from adding fresh water that isn't pH adjusted. If the plants are drinking a large percentage of water daily, a larger reservoir may stabilize things better. Also you should expect pH changes (especially when replenishing lots of water) when adding water that isn't pH adjusted first and/or the pH of the fresh water you ad back is off by more than 1.0 or so.
As an example if you are replacing 5%-10% of your water daily, I wouldn't expect to much fluctuation in pH unless the pH of the water you replace it with was far off (1.0 or more). However if you are replacing 25% or more daily I would expect to see a lot more fluctuation, even if the pH of the water you are replacing it with wasn't that far of. Simply because if your replacing half of the water volume every other day, or 100% of it within 4 days, the pH is bound to change much quicker. Unless the water you replace it with was pH adjusted first of coarse. But in order to be able to compare EC/TDS/PPM, and/or pH readings/changes. You really need to mark the starting water level in the reservoir first. Otherwise you cant really be sure what your comparing one reading to another.
I'm also unclear about weather you use a "pH drop test kit" to verify the readings your meter is giving you. Meters need consent care, and can give false readings at any time. If there is ever a question as to weather the reading is corect, you should always verify it by using a pH drop kit. That's always the first place I would start, and the reason I don't even use a
pH meter in the first place.
What I can tell so far is, I don't see a problem with the
nutrients your using because the GH line of nutrients have pH buffers. Also the RO water is fine because the RO system takes out as much as possible of the minerals that could cause pH to change. However it still needs to be pH tested and adjusted before adding. 7 to 10 days between
nutrient changes shouldn't be a problem either, however that depends on water volume to plant ratio, and how much the plants are drinking daily. But "under normal conditions" quick pH fluctuations generally indicate it's time to change nutrients (especially when plants are large and drinking a lot of water). GH pH adjusters wouldn't be a problem either. It sounds like the algae growth is not enough to cause the type of fluctuations you say your getting. If your flushing the system adequately, you should be able to flush out most of the built up mineral salt deposits that could change pH, as well as cause EC/TDS/PPM to gradually rise. If you have any concerns as to weather flushing with straight water is, or isn't working well for you, you may want to try using some
Flora Kleen too.
How I flush my systems. First completely drain the system as best I can. Then empty the reservoir, quickly wash it with soapy water and rinse well. Then I take apart the
pump and wash each part with soapy water as well. Then I put it all back together, fill the reservoir (about 1/2 to 3/4 of the way) with fresh water, I just use regular tap water to flush with because it takes to long to get RO, and it will just be dumped out anyway. I let it run for 15 to 20 min, then drain and dump. I do this 2 to 3 times to get as much of the residual nutrient solution out of the
growing medium, and system as possible before I flush. Then I fill the reservoir with fresh water again (all the way this time), and let run for 30 min or so, then drain and dump. I do this a few more times, usually letting it run longer each time. Of coarse I often alter how long I flush depending on how much daylight I have to work with that day (once it gets dark I cant see outside very well at night).