Ph Problem!

Hey everyone, I'm a long time lurker first time poster and I have a little problem.... I just started my first indoor aeroponic grow and for some reason my reservior reads about 8.0 ph.... I have one of those instant read ph monitors and it keeps jumping all over the place. I have a ph down solution and i tried using the recommended amount, then double that, etc etc and it still wont change. Any suggestions? btw I'm growing in a small closet with less than average ventilation, the sprouts look healthy. thanks everyone!

I went out and bought a new ph tester tonight to double check and found out the other tester was defective. It kinda sucks that this had to be my first post but oh well.

if your ph tester can read a 2.0 you have a great one. use lemon juice (ph=2.0) vs tapwater (ph= 7.8 to neutral) for calibration

all Ph pens need regular calibrating ... about once a month is good ... and always give it two points of calibration to make sure its reading up and down accurately ... buffer 4 & buffer 7 ... E.c meters rarely need calibrating usually just need to be kept clean (jiff is best to clean the prongs) but its good to check every now and then for your own peace of mind ... cuz if your pen tells you lies it can cause all sorts of problems ... man chemists and shit calibrate their Ph meters everytime before they use them .... ooo also dude try not to use too much 'Ph Down' if its 'Phosphoric acid' because it bonds with calcium and turns into Calcium Phosphate which is an unabsorbable form of calcium .... calcium deficency looks sort of like rust spots on the leaves ... It will only happen if you use heaps ... and man the secret to success is mastering your enviroment so i would focus on that ... Air Quality (temp,humidity,freshness, movement) - Water Quality (ph, e.c,temp,freshness) drainage, and light quality ( young bulbs ) ... it makes a massive difference if you flush your pots out thoroughly with a trigger shower nossel on ya garden hose - once per week in clay cuz it does not retain much moisture ... once every two weeks in other medias .... fully hose them out with force and let all the shit run down into the resevoir (or just let it freely flow out the bottom if you run to waste) and start again with fresh nutrients so you know you have the correct ratios of all your nutrients at all times and also prevents any toxic salt build up ... their are heaps of reasons why your ph might be rising quickly but yeah a dodgy Ph pen will do it ~