I finally was able to afford a $60.00 ph pen and calibration solution. Have compared it to the aquarium tests I've used for years, and they are only about .1 off from each other. (close enough)

Soooo...

I use GH ph down, and mix the water to exactly 7.0. I then add my Fox Farm flower nutes. (Big Bloom, Tiger Bloom) at 2 tsp/gal each. According to the ph pen, the ph drops to 6.3. Tried to use the aquarium kit, and color from the nutes was messed-up enough (cloudy) to not be able to really tell.
As the aquarium kit does great for clear water, will still use it to double-check my pen and for a back-up.
Was amazed that the ph would drop so much, just adding nutes to the water.