The first mod I made to my pond last summer was a Hudson valve. It isn't permanently installed, yet. I just screwed it to the end of a hose and dropped it in the pond, adjusted it to the correct level, and put a rock on the hose.
I had a leak in my waterfall that dropped the level about 1/2 inch per day, and the valve fixed that problem. Then I was able to address the leak on my own schedule.
It worked all summer, and it was great not to have to worry about the level.
This summer, I will actually install it, something like Jerry did.