Power head or bell?

So what's the word out there? Is it better to collect CO2 in an upturned
half bottle or to run it through a power head?

I'm using 2 DIY yeast bottles that I change in alternate weeks so that
neither of them are ever running longer than 3 weeks. They are running
through a power head. Seems like there is lots of CO2 going in to the tank
(a 48 gal) -- lots of bubbles coming into the power head about 1 per
second. However, whenever I measure pH, which is 7, and KH, which is 40 ppm
/ 2.2 dh, and then use the chart, I see that I only have about 6 or 7ppm
CO2 in the water. I'd rather have it at 15 ppm. How about THREE CO2
bottles? :)

There are oxygen bubbles under the plant leaves and lots of (what I assume
is) CO2 bubbles floating just under the surface and coming out of the power
head output. How come all this CO2 never lowers my pH anymore? It used to
go down to about 6.5 but now stays about 7 all the time. Could it be that I
have too much surface agitation? I do have some surface movement but I also
have a glass cover on the tank.

Another thing: I find it difficult to judge the pH accurately with the
colour bars on the Tetra test kit box. Maybe my eyes are getting old!! :(
Being sure if the pH is 7 or 6.9 or 7.1 is almost impossible as far as I'm
concerned. Are there kits that don't cost an arm and a leg that have a more
sophisticated way of showing pH levels? Or does everyone just make a guess
within a point or two?

in sunny, warm Vancouver.