ph problem whit distillie water goin up fast

Hey i got a little problem whit my distilled water i got a 3 gallon kit plants are doing really good but my ph keep goin up i stabelized it 30 min later back to 7

i use a basic powder ph down and yes i desolved it in hot water before putting in my tank

Distilled water has no buffering capacity; every time you add something to change the pH (typically phosphoric acid to lower the pH, and potassium hydroxide to raise it), it'll overshoot.

Add back in 10% tap water, or add a buffer like citric acid- or use a buffered nutrient formulation before adjusting the pH.

Keep it simple.....Use tap water and 3-5ml H202 per gal, let it sit out overnight and you're good.
It's also good to note that smaller volumes of water usually have a harder time maintaining stable PH. I constantly had to adjust my ph 2 times a day when using 5gal buckets. I switched to 10gal tubs and my ph stays stable for about 4 days or so before it starts to creep up again.
Make sure you let that hot PH water cool down to room temp before you put it in your rez, temp changes also effect PH.
Good luck