Just got my ph pen this week, and given I'm in a never ending battle with bba I thought I should double check my c02.

Here is what I have:

kh: 5
ph after degassing: 7.5
ph 3 hours after lights and gas on: 6.9

From what I can tell (https://aquariuminfo.org/co2calculator.html), that puts my c02 at around 18 - 19 ppm.

This is a touch on the low side? Especially given I'm scarping off bba every other week?

My tanks is 75 gallons, pretty heavily planted, and there is a lot of surface movement (a canister filter and a big hob filter).

I'm pushing a constant stream of bubbles into an inline diffuser. Am I hitting a ceiling due to the surface agitation near the hob filter? Should I even be concerned with this?

Plants seem to be growing OK. Here is a pic. You can see the c02 smoke being pushed out.

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Surface tension will degass the water a little faster. I had my lily pipe half in and half out once and it changed the ph by something like 0.3 compared to normal. It wont cause a hard ceiling though.

I'm a little confused by the way you are wording your post though. Have you tuned your co2 via a bubble counter or not? That's the first step. Increase it a bit until you lower your ph to your desired target, which can be a little lower than it is now to get to that magical 1 point drop.

Right now I have my c02 going through a bubble counter, and I'm counting about 6 bubbles per second. From the bubble counter the co2 goes to a diffuser inline with my filter return. If I turn the co2 up much higher then I get occasional large bubbles of gas out of my filter return, instead of a fine mist.