ppm low but ph keeps dropping I NEED ANSWER PLEASE

1.bubble bucket 2. hydroton 3.r.o water 4 nutrients dyna grow, maxi grow, bc 4. strains skunk ,ecsd,og
ok now that we have that situated the problem is I been having so much trouble keeping ph stable .I got a ecsd @105 ppms with 15 main tops in its 3rd week of flower I also have a super skunk @290ppms bigger and more tops then the ecsd . I thought the rule of thumb was if your ph is dropping and your water level is lower then its too many ppms .these plants are fucking giants the skunk drinks a gallon a day.the ph will go from 6.3 to 4.9 in 24 hours . I just got a ppm meter today but top off buckets yesterday with nutes and checked the ppms and when it said 290 i was astonished. plants are healthy anyone have any input??

Not trying to be a smart ass but are you sure you are reading the EC correctly? Also are you using any bloom boosters. Yes your right i think (newb here lol) but in my limited experience ph drops as rez ratio gets hotter. I run EC at 1.2 max in a 30 gallon rez and it takes a week in flower for ph to go from 6.1-.2 to 5.8-.7. When that happens I refill. I used a bloom booster once (couldnt resist lol) and ph plummeted.