First off if you want to do hydroponics right stop fukin around trying to make whatever shit you can round up localy work & buy proper ph up & down from an online hydroponic supply.
I read where you said you dont have a hydro shop in your country but you dont need one anyway,just order the stuff & in 5 days its at your house,then you have eleminated any issue with weather or not the chemicals you are using are the proper stuff to use.
Next when you loaded your net pots with hydroton did you clean & soak the hyroton ?
You mentioned you did a grow last year,is the hydroton your currently using left over from last years grow where it was used in the previous grow now used in this grow a year later ?
If so much of your ph swings are comming from the hydroton,used hydroton holds nutrients so if the hydroton your using now was used previously,then put away without proper cleaning,then reused without proper washing,rinsing,soaking & ph'ing than that is the bulk of whats causing drastic ph swings.
Most of all guy you have to get the proper chemicals for ph,just trying to make other stuff work isnt advised,all the time you spend messing around trying to get stuff to work is just more time the plants are not in a healthy enviroment,the longer the plants are unhappy or sick the less bud they will produce come harvest.