Been a couple of pages since I read this.
First Carlos everyone will agree that your english is much better then mine. Afterall, english hasn't been spoken in America for years.
As I understand it your bucked has little to no circulation and not bioload yet has a pH of 8.0. As opposed to your planted tank with circulation and bioload treated with prime and a very low pH.
KH and Gh are unknown because your LFS didn't have the test kits.
Just some observations which may or may not reflect the totality of your situation.
I find is disappointing that your LFS who sells api ammonia test kit does no also have the api combined kh/gh test kit.
PH is a function of both kH and carbon dioxide. IME carbon dioxide has a much greater effect than KH. I have had both marine and freshwater tank with kH values ranging from 4 degrees to over 20 degrees and the PH is 8.4-8.8 with the api high range test kit. Don't get me wrong kH is important and does seem to help limit the nightly pH drop. But the just before lights out pH values were those high levels even with tanks that had peat moss in the substrate and which originally had pH values of 7 or less. But after things got through the initial cycle the pH would rise to those levels.
I feel your experience is reflecting that fact. The bucket with no circulation and no bioload is probably low in carbon dioxide therefore has a high pH. And the tank with circulation and bioload has a much higher carbon dioxide level. Especially with the prime dosing.
Prime does detox (lock up) ammonia but in the process reduces oxygen levels and lowers pH. Plus the ammonia still test positive with most test kits. The danger is you treat prime to lock ammoina, still test ammonia, add more prime, etc,etc,etc. Meanwhile you reduce more and more oxygen in the tank.
With the plants thriving I would recommend you stop water changes and stop treating with prime so the plants will be more effective. I feel if you do that any remaining trace of ammonia will drop to 0 and pH will rise as carbon dioxide is consumed by the plants.
And of course you should only measure pH just before lights out. If pH is high at that time, it reflects that the tank (plants) is fully consuming the carbon dioxide being produced by the fish.
At that point the fish should be extremely healthy in the low co2 environment and the plants thriving also.
And they will just basically take care of each other.
still just my .02