That's a really good question on the cause. Filters had been getting cleaned, I know I did a complete water test several weeks prior to this and generally over winter it is pretty stable.
One other thought is that I have been using a commercial root tab made from white clay and organic fertiliser. I do wonder on what the organic fertiliser is exactly and if that may be a problem. No one else seems to report any issues so I've put that down to coincidence so far.
At the moment I'm guessing it may have been a few little things that added up but honestly don't know.
Tested the tank ph today with the co2
ph probe (6.7); api test (6.8); digital ph pen (6.9) and seneye reef (7.5).
Plan to try re-seating the seneye ph slide chip and clean the housing next water change. Grrrrr. I'd love to get it working as could see ph across the tank.