You can try the noise/scratch removal filter but these don't work fantastically so well for backscatter from particles in the water. I normally use this filter as a first step to partially remove some particles, and then clone away the rest. Hope this helps!!
If you are not already doing it, a better way is to illuminate the tank from the side/diagonal so that the backscatter is not reflected back towards the camera. Your flash will obviously need to be positioned off-camera.