In ponds where filtration units do not allow a build-up of mechanical debris, I have never seen Hikkui appear. It is always in ponds which have little or no means of removing debris from under the filter media trays efficiently and regularly that problems with Hikkui are experienced. All we are trying to say is that coral easily allow the build up of debris and is more difficult to flush from the system. You simply have to wash them carefully to remove all debris. Hence we always do not recommend the use of coral.
Many people use sea shell in their last chamber to control pH and is very effective.
To help to control pH with coral, you will only require a small quantity to be place at the last chamber where the water is already quite clean. You definite do not require the whole chamber to filled up to control pH. Hikkui is not limited to only Japan but also occur in Malaysia. Robert Yap once told me one of his kois has Hikkui. Peter Waddington also mentioned in his book that Hikkui problem is quite serious in UK in the late 80's. I also think that it is not a matter of proper disinfection of the coral.