Instead of cutting off water to farmers to save some fish why not just start breeding programs?

That depends on the fish species you are talking about. First off building a lake is a huge undertaking and really can only be practically done by damning a river creating a resevior. There are many potential issues to overcome there, ie relocating people, wildlife, and what do you do about the native fish? Now if you are talking ponds, how to you maintain a large population of fish without having a continuous food supply? If you supply the food, what do you do about the accumulation of wastes. What do the fish eat, other fish? Where do you raise those fish and what do those fish eat or require to breed? It isn't that simple of a problem. Why not just make it rain more in that area so the farmers don't need irrigation? I agree with you that aquaculture is a viable resource for fish production but it isn't always an easy task because different species need different things to survive and breed.