I have never heard of this idea before,we use plain water!

Dogs still don’t drink water with vinegar in it, it still sits and ‘grows’ black stuff. So I stopped putting it in. Add a little to their food instead, they eat that

I never heard of doing this-Interesting

Maybe you are putting too much in the water. I don’t know whether the humdity level of your area would affect, how much dirt and dust is in the area you are living in. I wonder what is in the water when you get it, whether it is filtered in a commercial plant at the reservoir before it reaches your property, whether you are using water that has been through a household filter or water softener. Depending on how much water your dog drinks I would be at least rinsing the container every day, perhaps more than once.
I am also wondering what the container is made from – whether it is leeching something from it into the water. I reckon that would happen with aluminium and probably some types of plastic.

That happens with my pets water but only if the water is not changed every few day. But this is with plain water and nothing else added.. I am surprised that its happening so quickly all i can suggest is to Try a new bowl that the water is in..

It should be added to their food . It can have health benefits.

I first read of it being good for fogs that have tear stains, supposed to keep it under control,a little. I bought it and then found out it has multiple health benefits for my dogs but also for me. I’m just not sure about why it’s leaving the dark build up the sides of their water bowl

I wouldn’t know. But apple cider vinegar is indeed very good for dogs. My vet told me to apply it, mixed with water, to my dog (who suffers of allergy) and often scratches and hurt himself .