In light of your somewhat alkaline conditions, a much less amount of alum should clear it up than would be required using gypsum.. Probably wont have to use enough to make it too acid and your soil should put alkalinity back into it. Do lots of reading about how to apply alum in a stocked pond so as not to stress the fish abnormally. You can always have a little hydrated lime to help neutralize the alum. Have a testing kit handy to monitor the water. I wouldn't have a problem DIY, but if you feel uneasy you may want to hire it to be done.
