Proper fountain maintenance will keep your pond free of algae and limescale.
For green water, you can use the algaecide or a UV sterilizer. Because there are no animals in the water, treating aggressively with algaecide is possible. The algaecide will rapidly kill off algae in the water fountain. UV sterilizers will also work to curb algae growth and prevent green water outbreaks.
For string algae, manually remove as much as you can before treating; that prevents the string algae from decomposing in your fountain and clogging pumps.
Blue dye (pond tint) can also be used, but it may stain some fountains. Test in an inconspicuous spot before use. Pond tint is very effective at keeping algae from getting the sunlight it needs to thrive.
Limescale remover should effectively remove minor deposits of limescale. In extreme cases of large limescale buildup, take a half cup of limescale remover and heat until hot (don't boil). Use an old washcloth — something you don't mind being stained / damaged — and let the washcloth soak up the hot limescale remover. Put the washcloth over the limescale buildup and let it sit for 10 minutes. Scrub the fountain surface area gently with a soft-bristled toothbrush to break up the buildup. You can use a stiff nylon-bristled brush on durable fountain materials, like concrete and cast stone, which can stand up to more vigorous cleaning. Before doing this, read the instructions that came with the fountain to insure that there is no special coating or treatment that may be harmed in this process.
To clean pumps, use a toothbrush to remove as much buildup as you can manually. Then soak the pump overnight in a dish of limescale remover or vinegar.
It's important to always make sure the fountain has enough water. Oftentimes, when most pumps are no longer submerged in water, the motor will continue to spin causing the pump to burn out.