pH swings

I typically experience a swing of 1.0 unit on a daily basis. ie pH 7.0 just before lights on and 6.1 2 hours after lights on. This is a hundred fold increase in acidity!
What swings do others have ? My Co2 is linked to the lights.
LAKA

 

I have surface agitation 24/7 using a powerhead on my 180g tank. There is a moderate rippling but no water breakage along 3/4 the surface area of the tank. Tonight i decided to up my CO2 as my 4 degree drop checker was still blue. pH dropped from 6.4 to 6.1 half an hour after increasing the CO2 levels and immediatley all my angels headed for the surface with heavy breathing.
I thought it was CO2 overdose but KH sln. still blue! (I checked with a KH kit that changes to yellow at exactly 4 degrees so i presume my ref. sln. is OK).
I then suspected the large increase in acidity may have caused this. I backed of CO2 slightly , pH lifted to 6.4 , breathing rate back to normal.
I don't think it is deficient O2 as no fish stress when lights out.

My CO2 turns on with fluorescents at 12 noon and metal halides kick in at 2pm and turns off with all light at 10pm.

So how am i going to increase my CO2 to the desired 30ppm without upsetting the angels?
LAKA

 

Is it possible my CO2 levels did skyrocket which was picked up by my Ph meter indirectly almost instantaneously ,but not by the drop checker as the gaseous diffusion process is much slower to react??
LAKA