Instead of meds, have you thought of putting some potted pond plants in there? The green water looks to be due to an algea outbreak (too much nutrients). The pond plants will eat up those excess nutrients and the water will get clearer for you. It may take a couple of weeks but it does mean that is one less chemical you'd be adding to the water. One plant that comes to mind which I believe would help would be water lilies.
If you are dead-set on adding a chemical, you could try This
If you are dead-set on adding a chemical, you could try This
