huh. well, if you feel it's ok, and everything seems to be working well for you, then it's probably fine, but I would still be a bit wary of that drastic a pH swing...
I think it has less to do with the O2 then the CO2 - I forget the chemical formula, but CO2 dissolved in water tends to form an acid (carbonic? I can't remember, but it's why rainwater tends towards acidic (5.5ish pH if I remember?)), so any agitation (like from the output of a HOB) would push the CO2 concentrations towards the atmospheric amounts (hopefully much less, if not, open a window or something...), and lessening the amount available to form that pH-lowering acid.
anyways, do you run your CO2 at night? and, have you checked night vs. daytime pH?
I'd also consider seeing about some sort of auto top-off mechanism using distilled/ro/di water to compensate for evaporation. this could be as simple as filling a 2-liter bottle with water, and mounting it upside down so that the opening is under the desired water level. kinda ugly, but pretty cheap and relatively easy to set up. and if you do at all regular water changes, you could probably just use tap water.
O2 will raise your Ph and Co2 will lower Ph (This is a good Reference Link)
Rain water should be neutral since it is one of the purest forms depending on how many factories you live by.
I run my co2 24/7 The reason why the water level is low all the time is because I always take water out of it for my plants and other stuff. I change my water everyweek enough not twice a week with tap water, depending on the tank. Dont really wanna hassle with RO/DI been there done that gotta keep it simple at the moment. I do have alot of evaporation due to a space heater, which was alot cheaper than buying and powering seperate heaters of all my tanks in my fish room. Im not that worried about evaporation. Alot of the cause is the hardness of my water. Since it is moderately hard it doesnt take much to buffer it.
Even though I am still moving the surface with a powerhead as well as the modded HOB the ph has remained stable at 7 day and night test. I have my power head low under the water so not as much oxygen actually gets churned into the water. I have finally found balance
The autofill bottle idea is a good concept and has me thinking....Theres always ways to make it look better.
Really the PH swing isnt that big of a deal or a problem. Fish deal with PH swing quite easily. They do it as well in the wild.
My tanks with pressurized Co2 run the PH during the day around 6.8 to 7.0. The tap is 8.3. At night when the Co2 is off the Ph swings up to 8.0ish.
Never any issues for the fish.. Shrimp on the other hand I think have issues easier. But most fish will adjust and tolerate it fine.
Thats why I changed it, my fish didnt mind but my Super Tigers arent a fan of the swing.
I used to use clear CD case tops and taped it behind the hang on back entrance. The thin cd cases also work and you could use the black area instead of the clear. Yours works too and should be just fine.
Hmm thats a good idea too. Would of saved me the time of fabricating my own, even though I kinda of like that part
Even though that plastic isnt the most fun to cut.