I've got a Drinkwell Pet Fountain. It's porcelain, not metal and WOW! Does it get nasty dirty! Maybe it's my water quality or cat spit but it gets a ring of crud around the bowl and over the lip of the tower.
The best part is that you can scrub it and soak it in vinegar or bleach and it doesn't do a danged thing! Not a bit of difference. It's like that pet fountain crud really means business.
One day, while scrubbing at it to no avail, I scraped at it with my fingernail and voila! There WAS smooth porcelain underneath! It could be scraped off, but with what?
Suddenly, my mind replayed an old commercial in my head.
"Don't scrub your toilet with a pumice stone!"
And seriously, who reaches into their toilet and scrubs it with a pumice stone? EWWW! But a pumice stone would totally get the crud off the cat fountain, right?
BING!
Why yes, toilet bowl cleaner cleans it right up, in seconds, no scrubbing!
1) OF COURSE, by all means, wash again with detergent, get any toilet bowl cleaner off of it before you give it back to your pets. Better safe than sorry. And duh, of course, only use this on the porcelain parts, not the pump and filter.
2) This is acid, wear gloves or don't touch until it's rinsed.
3) Don't get toilet bowl cleaner on metal-like a metal sink. It etches. If you've got a big metal sink, clean the pet fountain in the bathtub, like I do.
And there ya go, cleans a cat water fountain just like new!
