Having trouble getting the right Ph level

[Sith]

I grow in soil, but I have a hydro setup for my clones, a topfed system with net pots, I have a water tank beneath and I recirculate the water 24/7. I also have a air pump to oxygenate the water. So far, after about a week they seem to be rather ok, but no roots so far..

I have an old analog Ph meter, but I'm having trouble getting the right Ph in the water.
After having tested with 1l of water, the result came out like this:
Ph of water without nutes and Ph- fluid: about 6.9, ppm 150
ppm of water after adding some nutes: 594, the Ph stayed the same, 6.9
But after adding 1ml of Ph- fluid, the ppm level was raised to 1308!
I've always read that the right thing to do was to add Ph- fluid after you have added nutes, should the ppm level really be affected that much from adding Ph- fluid?
This way, I'll often have to add some more tap water to lower the ppm level in the end!

And the Ph meter also fluctuates wildly after adding Ph- fluid, from 5.5 to 6.9 in this experiment, just by tapping it to the sides of the water container, or by moving it from the bottom of the tank to the top.
When I take a small cup of that water and then test it, the ph level seems to be nearer 6.9..

I would appreciate some info if I'm doing something wrong here, and how to do instead, maybe I should get a more accurate digital Ph meter? Maybe that's the problem?
Any help would be appreciated!

The plants seem to be not perfect, but quite ok so far, maybe they aren't so sensitive after all?
What ppm level should I have for my clones?

 

[AshSoul]

Hello. Adding pH down should not raise the ppm at all. Are you mixing the reservoir really good after adding nutes/pH-? You're right tho, add nutes first, then correct the pH if necessary. 594ppm is way too high for clones IMO. I'd shoot for 200-300 range, that includes your 150 starting ppm. I prefer tap water that has sat overnight with an airstone in it then add CaMg til it's around 250ppm.
I would make sure both meters are calibrated in known reference solutions .. 4.0 and 7.0 for the pH meter and 2.77 EC for the ppm. I had the Hannah pH meter before that cost around $30 (US) and it would have wild fluctuations testing tap water. Once I added nutes tho it would read fairly quick. But when I calibrated it every 6-8 weeks it would need a lot of correcting to be fully calibrated. The ppm meter tho(also Hannah) was rock solid. Didn't sway nearly at all between calibrating(same frequency, more or less). I dropped it in the water one day and it wasn't waterproof lol
Now I use BlueLab ppm and pH meters ..