Starting with 10-20 out of the tap is a bummer, but for it to go that high... That seems crazy for a planted tank.
In your photo of the 75 it looks lightly stocked. Is that the case in both?
Does the CO2 affect it maybe?
What's your filtration?
Also, I've read from several people that there can be falsely high readings if you don't shake the hell out of the #2 bottle. The instructions say I think for 1 min but I do 2 min now based on a recommendation from an experienced member. You might be doing that already, but I thought I'd throw that out there too.