Update:
I found the issue!

Stupidly, I had listened to the advice of YouTubers and Amazon reviews. In my sand filter, I had used the woolen balls. Even though I had been performing the cycle and flush, dumping the water, the woolen balls were not clean, they were green and completely swamped in algae.

I took them out, rinsed, sat them in some diluted bleach and rinsed again. I cleaned the filter thoroughly. Upon putting it all back together, throwing in some shock and giving it a few hours, along with a 1/5 pool water dump and replace, it's looking clear again.

Thank you for all of the advice. I'll move to sand in the near future so the rinse and dump cycles actually work.

Original message:
I wasn't sure which group to shove this in, I went for home improvement as this needs serious improvement.

To be honest, with how green it is, maybe enviro would have been even better.

I have an above ground 5800l pool, which I put up during the summer for the kids. This year I decided to go with Bromine in a float. I'm using the standard slow disjsolve capsules from Everclear. I chose Bromine as one of the kids has quite sensitive skin. I started out with the winder float, 6 tabs slowly dissolving. I gave it a couple of days and realised I didn't have strips to test, so I ordered some.

In this time, with lots of abuse, a very warm season for England and probably half a metric ton of sun cream, the water was looking a bit cloudy. No biggy, the great Flow Clear 800Gal/h sand filter would be helping... It didn't seem to be. The water went from cloudy to slightly green at the bottom. The algae was arriving.

I dipped a stick: bromine didn't even register. Hmmm. I ordered a second float and some algaecide to attack the issue. Two days later, we have 12 tabs in two floats, enough algaecide to clear the Amazon river (I jest), and I leave it to clear. The following day, it's less cloudy, still green tinted and bromine still does not register on the sticks.

Over today's use, it's gotten worse. I've ordered chlorine tabs instead because the bromine is not working. It should have worked. Does anyone know where I went wrong?

Tomorrow, I'll dump half the water (over 2000l down the drain), change the tablets over to chlorine, throw in some more algaecide and let the pump keep on pumping. Hopefully, as it refills, I should get some nice clear water that doesn't have a pH of near 9.