you shouldn't measure the solution until quite sometime after you add anything to it , water , food , ph
as it takes time for whatever to fully disolve into the solution, i normaly wait a few hours before taking readings
when i first add the food if i take a reading it will be about 1.6 EC after 2-3 hours it will be 2.0 - 2.2 as all the food is able to dissolve the reading goes up
since the ph down is most likely to be phosphoric acid then it will add a tiny amount of
phosphorus which is one of the main NPK's so its possible the ec/PPM will rise by a tiny amount when you add ph down
5.5 -5.8 is low imo, i would keep it slightly above 6, when the water level goes down if you are keeping it at 5.5 it will go even lower
do not worry too much about keeping the ph at a constant number, between 6.5 and 5.8 is fine , try not to be adding the acid anymore than 1 time per week
then reduce it to 1 time every 2 weeks .. you will find that the plants will grow fine
it is more important to keep the ec/ppm under control, so the plants are always getting the maximum amount of food they can handle at all stages of growth
sometimes when my plants are having a big growth spurt i am adding food with almost every water top up, 2-3 times per week to keep it at 2.0 - 2.2 EC
i personally do not check the ph or use any ph meters but i do add ph down every few weeks
peace 
