Assuming that your test kits are all accurate, my guess would still be that it is from excess CO2 if your not adding any chemicals besides kalk, although getting more fresh air into your skimmer should have helped. Have you read this article?
Chemistry and the Aquarium IMO, it is the best article on the web dealing with pH in aquariums.
Take a bucket of water out of your aquarium and place it outside for a few hours with a powerhead or air pump circulation the water and rippling the surface. This will drive off most of the CO2 in the water. After you do that for a while, retest the pH of the bucket of water and see what you get. If the pH in the bucket is higher, you still have excess CO2 in your tank's water.
BTW - I have battled pH in the range of 7.65-7.8 in the past and the corals showed very little sign of stress. I know that it isn't anywhere near ideal, but, as you've already seen, your corals and fish can handle a low pH.