When we use the ozonator to agitate the water it creates extremely fizzy water - it actually looks white - and creates a very strong odour which catches in the back of your throat - even though the bromine levels are low to medium?? PH low (seem to struggle getting it increased). Do we need to empty and start again?? Dont know if this has any bearing but as we live in Spain we havent used the heater for over 3 months - water stays at a constant 31 degrees??

The first chore is to balance that PH level. Then I would shock the tub. If it's been a while since the tub was drained and refilled, I would recommend doing that first. Also don't forget to soak that filter!