All concrete will leach alkalinity into the water for a period of time. This tends to drop off after the first month or so. For instance, I couldn't even keep fish in my aquarium for the first couple of months. I had to do lots of daily 100% water changes and towards the end, I had to test with a single gold fish watching closely to see if the fish was showing signs of stress. Eventually the system stabilized- pH, Alkalinity, & other water parameters all test at normal levels now & have for over a year and a half.
I do not think the concrete background is the source of my problems. Granted, it does provide some challenges with respect to providing a surface close to the light source for algae to grow on but algae isn't my main problem. So far, it looks like insufficient light was my primary problem, aggravated by some other factors like poor reflectors, & a very tall (30") aquarium.