The issue is not them running low on water, but needing to dump the water and change it because it grows red algae or gets slimey after a few days, right?
Drop a peice of copper in the water.
I do this for a two-gallon water-jug my cat uses. I use a big old british copper penny, the size of a silver dollar. No copper, I must change the water at least weekly or it gets slimey. With copper, I can wait 'til it's nearly empty and the cat has gotten fuzz and stuff in the drinking bowl part and the whole thing is due for a wash.
No, it won't put enough copper into the water to harm your animals. This phenomenon is the reason for copper plumbing. A length of leftover copper plumbing pipe will do you perfectly. A foot of new copper pipe will probably set you back about $3 at the hardware store.