A friend of mine has the very same problem.  She lives a few miles from me, but all her's died this summer, while mine, which came from the same batch and same tank at the store are still doing great.

Look at the warning on fly sprays!!! Some of them don't like fish! One of our horses loves to play with his nose in the water....guess what! Dead fish! It wasn't froze and not hot!

Go to Home Depot and buy a jar of the pool chlorine tablets. Break off about 1/4 of the tablet and drop it in the tank. Been using them for 5 years in my 100 gallon stock tank, I think my horses like the taste of the chlorinated water better than plain.

My horses, unfortunately, wont touch the cholorine. I find that those wonderful Rubbermaid 100 lb grey water troughs we all use are the problem. Great in theory but algae magnets in reality.

I recently turned my rubbermaids into troughs to soak my hay (long story as to why that's a good idea, esp. in sandy areas). I switched to those clear plastic square storage containers you can find at the dollar store. For some reason they do not attract algae and the are really easy to rinse out everyday. (If you have a large herd of  horses, try lining up more bins side by side.) I've just found there is no solution for algae when it comes to  rubber or metal troughs.