I also was going to suggest plants, but I wonder if your fish will eat them all? I have a lot of algae in my small ponds until the plants get going, then it gets really clear. How deep is your pond? What kind of fish do you have? 50 sounds like a lot, but I don't have any, so I'm no expert.

I tried a few plants last summer probably not enough they didn't do anything and the fish seem to be a mix of koi and goldfish

There's also a lot of plants in between the rocks and such and a tree blooms and shades a portion of the pond

Have you scooped the dead vegetation from the bottom? Stuff will have accumulated over the past 4 years.

My pond always turns green in the spring, but like Catherine mentioned, it eventually clears up. I do filter mine and have running water so your mileage may vary.

I'm afraid to stir things up, I've read by doing that you can release stuff and kill all the fish off, if I'm mistaken I will definitely try that

I don't know anything about bigger ponds, but it's hard to believe that the green color (algae) can be good for the fish. It certainly has to be decreasing the oxygen, don't you think? Are you sure all those fish aren't carp? lol!

We hear about the 'stir things up' issue here all the time over the years, but people seem to stir things up while doing all kinds of things and we don't normally hear about fish death from specific activity. The argument seems reasonable but doesn't seem to be born out from experience. Perhaps you could do a little at a time; it seems you could only do it that way considering the large size.

So get in there with some waders and start shoveling we'll see how it goes

I think the first thing I'd do is figure out a filter/pump/oxygenation plan. You could have an external filter which would be easier to get to and clean out. You could have a spray of water in the middle of the pond to help oxygenate it, and get more (not too invasive) plants in there.
I think you were just joking, but I wouldn't start shoveling yet...........just go with the first things I mentioned. I understand your fears though. I know when I cleaned out my little pond, I stirred up a brown mess, but everybody (frog and tadpoles) seemed to do okay. Bigger fish probably have bigger needs though.
I wonder if you could get some of the water out and save it, and then catch the fish and keep them in another container until you did a fair amount of the cleaning?
You didn't say...........how deep is your pond?
Sometimes certain things seem really neat........until you have to take care of them. You might have to do a fair amount of work to get your pond healthier.......but then hopefully, it will be more balanced and you'll have less to do.
You might also google how to clear up a pond. I'm afraid this site isn't as active as it used to be. Don't forget there are a lot of other places to get good info!
Keep us posted about your progress.

I get the impression that you were hoping to do almost nothing. In one sense I think you can do that. I have a secondary holding pond that has some goldfish in it to eat mosquitos. It gets no treatment and it is ok. However, you would probably be better off with some water circulation although that is going to be difficult considering the size.

I have mentioned several times that a stream can be a great filter. It is essentially all that I have. I almost never do anything except clean out algae from the stream. Actually, I am thinking of removing and dividing my lilies. I usually do that every two years. That stirs up a lot of crud that gets partially trapped in the stream algae for removal. See my old link