Empty and scrub with coarse salt. Rinse well.

There are a number of cleaners that not only clean your fountain but keep it clean as well. They are completely safe for animals of all sizes. We actually sell them here:
http://www.serenityhealth.com/water-fountain-cleaning.html

Please feel free to give us a call with any questions!
1-888-881-4668
-Serenity Health & Home Decor

You can use a low dose (and check with the pharmacy first) of chlorine tablets. They dissipate quickly in the sun, but will kill algae build up. You can put it in at night when they are asleep and by morning presto the work is done...We use this system for our water basin and the pigeons keep coming back... so it must work!

We would recommend Fountec...it is safe for birds, plants and animals.

http://www.soothingwalls.com/fountec-fountain-cleaner.html

We offer free shipping and ships out same business day.

You can also use Bon Ami cleanser to scrub it out. It is a non-toxic product. And as the label says, "Hasn't Scratched Yet!"