Thanks again to everyone that have posted the nice comments, its appreciated.
I run my Ph at 7.5 and add Bicarbonate of soda to increase the Kh, water is very soft which leaves the fish open to sudden Ph crash due the the heavy feeding regime. I have only been using the bicarbonate for the past year, the water must have changed from what it was in the days gone by.What alerted me to the problem was one day I noticed the rays looked lethargic and their eyes started to mucus up, its easy to become complacent when you have had something as long as I have, you get into a pattern and stick with it,experience quickly points you in the direction of any sudden changes in behavior etc, its not until something seems a miss that you start testing the water like you always used to....every now and again we all need a kick up the backside!!