Hi Shelley, personally i would move the fish into the nearly cycled tank, even just till you get the main tank sorted. Sounds to me like there is possibly some old courgette or cucumber in the tank, maybe in the filter, my tank went cloudy when a bit of cucumber got stuck in the driftwood.
I would get the fish out, drain most of the tank, give everything in there a rinse (possibly leave plants), clean the filter and refill. Run it for a day or two and do large water changes each day.
Does it smell bad, as if something is rotting ?
Hope this helps.