DWC PPMs and PH question

This is my first DWC grow, I've been a soil gardener for a long time.

I've noticed the PPMs of my reservoir have been staying around 300ppm, while I replenish about 5 gallons of RO water a week to the 25 gallon reservoir. Everything seems to be going fine 2 weeks into flower and plants look healthy. The room is 80 degrees, humidity 50%, water temp 78, and PH of 5.7. I'm using the GH trio with CaliMagic.

Just wondering is this typical to not have to keep adding nutrients, or does this mean the plants are not feeding? I increased the nutrients to 350ppm a few weeks ago and noticed some light nutrient burn, so it seems these plants are happy with 250-300ppm.

Also, the PH drops about .25 every day, so I'm constantly adding PH up every few days to keep it in a good range. Is this normal too?

Have you done a res change out yet? I think it's time. With those water temps unless you have a good amount of beneficial bacteria or run a tip top sterile system thing could go very wrong very fast.

350 ppms at two weeks into flowering?...I've got to say that sounds like a pretty low dose. Also, I am shocked you've not had root rot issues with those temps, even with the application of bennies. But hey if the roots are healthy, then that's all that matters...but generally, a rise in ppm's is a result of one of two things:
Possibility #!- The nute solution has more ppm's than the roots are able to absorb, so they're consuming water faster than the nutrients making the ppm's go up in concentration.
Possibility #2- Due to osmotic pressure, if the ppm is not high enough, it will actually cause the nutrients inside of the roots themselves to flow outwards into your res, raising the ppm's in your nute solution. Running 300ppms, I simply don't see how you could be overfeeding them unless you plants are really small? I feed that much in early veg. Either of these scenarios would also account for your drop in PH. I'd do a res change, and then up your ppm's to at least 500ppm and observe for 24hrs. If the PPM rise and PH drop only worsen, then I am totally wrong here and your plants needed a reduction in ppm's.

350ppm is crazy low for 2 weeks into flower. I’m generally running 1000-1200ppm at that point. I run 300 ppm in week 2 of veg when my plants are 6” tall.

But if your plants are happy who cares, you’ll get a harvest, just not the optimal harvest your plants could produce. Obviously your yield will suffer this round but next time download the nutrient calculator for the GH trio and slowly ramp up the nutes as your grow progresses.

In high temps where the plants are respiring more you need to feed at lower ppm or you will burn them as you can see in the pics when the ppm was increased.

Always feed your plants based on how they respond feeding is different in every system.

You plants look like they are getting the appropriate amount of food and I would not increase the ppm.

If you are noticing stunted growth I would start looking at the roots but if they growing well keep on keeping on.

You are using RO and while I though calimagic contained calcium carbonate it does not. So without the ph up it doesn't have much carbonate/bicarbonate. Your ph up is likely nothing more than diluted potassium bicarbonate. You can buy this on its own extremely cheap.

One thing you can try is increasing the aeration of your water. This will ensure good gas exchange and if this raises your ph it's a good indication that your gas exchange was not adequate. You want the entire surface agitated this is where gas exchange happens not from the amount of bubbles you put into the water so you may need to add some airstones to achieve this.

Are you sure this PPM meter is calibrated? What conversion from EC does it use? 0.5? 0.7? I have never seen plants that actually have tip burn at such low PPM an dI see a little of that. Plants appear to be well fed so I question the meter.