Thanks for all the help everyone. I took all the water out and blocked off the light on the sides of the tank, removed the lamp and have taken to feeding our turtle outside of it for the last five days. There has been a huge improvement. For the first three days it stayed very clear (for the first time in a loooong time - what a relief!). Since adding the lamp back, a light tint of green has slowly appeared although not nearly as quickly of as severe as before. Seeing as we aren't feeding in the tank the biggest culprit seems to be the light.
I think part of the problem will be that the algae has been running into both our filters and remains in the tank - despite having cleaned it thoroughly without chemicals. We might look at a UVC to fend it off longer and keep up carefully monitoring what factors are having the biggest effect on the algae bloom.
I don't know why the bloom appeared in the first place seeing as our turtle lived in the same spot with same conditions for two years without issue. Never mind, I'm relieved to see an improvement thanks to your help. Whenever something goes awry you learn something new about your turtle and its habitat which can only help to you better caring for them.
Thanks so much.
