I have seen several people putting vanilla extract into their rat bottles, and I have seen some that look dark as soda.

I don't know if this is general knowledge besides people that bake often, but if that water is dark, it's too much vanilla!

Go ahead and take a sip of that water when you offer it to the rats. It's bitter and very strong.

All you need is a few drops to flavor their water. No more than 1 teaspoon per 1 liter in my opinion.

The water really shouldn't changed to a dark color, and at most be a light tea color. Any darker... and I wouldn't want to drink it either.