How to keep a water fountain from getting slimy & green?

I add a few drops of white vinegar to my bird bath. Birds, squirrels, raccoons don't mind a bit.

vinger

Also make sure it gets more Sun

Barley straw, available from pond suppliers, will keep the pond free of algae. You can also purchase liquid algaecide that won't harm birds or mammals, but use as directed.

The sun is causing your algae growth. Try adding pond plants for this purpose.

I use a little bit of vinegar too.

Small barley bales work to reduce this, I have the same problem, and my dog and birds go into this pond, I also use just a tiny touch of bleach, does not harm birds or dog, depending on the quantity of water, if the fountain only holds 1 or more gallons use only a tablespoon, and I would clean it and start it from fresh

When you clean the pump and tubing rinse them with vinegar and let them dry, this will help kill the spores that are left after cleaning, same goes with the surfaces of the fountain. I would check with pond suppliers if there are algicides that are safe for bird and animals if you want to go that way. There is no way to keep it from coming back eventually, it is from the sun, heat and moisture. If it was a pond, I would suggest getting a pleco for the pond, it is an algae eater fish that helps keep ponds and aquariums algae free.