IMO your best answer is to not get or find a fish to eat all the continual production of green growth because what will happen to the pond's overall water quality with all the manure produced by however many fish it takes to eat all that algae? More recycled nutrients from all that manure definitely means some form of nutrient surge and then more new plant growth and maybe even more growth that is now present.

Based on all the concerns and problems with the existing pond, it might be cheaper and IMO definitely a MUCH better end benefit to get quotes from some dependable contractors to dig a whole new small 0.1 ac pond and use the spoils dirt to fill in the old pond. This way you eliminate all the old nutrient enriched algae growing sludge and start anew wisely managing a new spring fed cold water pond; providing you still want a pond. IMO you should be able to do it for less than 17K and then have a brand new fresh pond with very minimal nutrient - algae problems.