I have corals so will not be able to turn off lights, will reducing them really solve the issue the schedule has not been changed or altered the whole time I’ve had the tank I set it to David saxby programme and left it from day 1
so the way to solve this issue would be either run carbon or uv, large water changes and reduce lighting schedule ?
the only thing I do not understand is what would of brought this on with no changes to the tank.
No one thing is going to fix it, a few small changes will, you can leave the lights and try the other options, but corals do fine without lights for 2-3 days, a reduction can help, you may have the lights on too long anyway.
Try the other steps first, you may need rowaphos or some other PO4 remover as water changes don’t really reduce PO4 that much as it will be in your rocks, removing the P.O.4 from the water will then allow the PO4 from the rocks to release.
If my tank, I would change the water to rid the tank of the algae, with filter floss, I use symec filter felt to get a good fine filter.
Try and remove as much as you can as if you leave it in there it is full of nitrates and PO4 so you make the problem worse.
As to how it got like this, a number of small things added together is enough, it is what happens so don’t blame yourself, tanks get algae.