Hi Leanne3, I had exactly this problem several years ago. I tried everything but failed, until installing an inline UV
light hidden inside a cupboard below the aquarium. After 4 days the tank changed from green to white bloom! But when I added a flocculent, and returned 30 minutes later, the tank was crystal clear! I guarantee this will work for you. I tried covering tank for days, adding daphnia inside a meshed container to protect from fish, tried adding a flocculent on its own. None worked. I have never had this problem again, but if I ever do, my UV
will be turned back on! Good luck.