Koi Pond Low Ph

Hi I just want to run a problem I have started having regarding my pond.

The pond has been running very well for 2 years. But I am now having problems with the PH dropping low and I mean low.

I have been trying to find the cause but can't. The pool is built in such a way that I can drain seperate parts, for cleaning. This I have done But I still have a low PH. I am at my whits end if this goes on for to long I will lose all my big fish to a slow suffication. Has any body had this problem and been able to deal with it.

The pool is concrete based and then tiled run for two years with no probs usually gives a PH of 7.2 11,000 ltr. I have cleaned out all the green algee that was on the wall and sides which was thick after cleaning checked the water 7.2 ok. As I opened up the pond you would expect the low PH to rise when diluted but no it stays the same. There is no rain water run off. If this last clean does not show an improvment then the only other thing I can think of is lime leaching from the concrete somewhere.