Older dog constantly drinks water
We have an older chihuahua, he is a rescue dog and when we got him they told us they thought he was around 10 yrs old. We've had him for about 3 yrs now and he's always been drinking water constantly and going to the bathroom in the house, he's also always been super thin and no matter how much food we feed him he doesn't gain weight. We've had him tested and the vet said there's nothing wrong. We keep him in his crate most of the time, and I feel bad for him, but if no one's watching him he'll go on the floor instead of letting us know when he need's out. Is this just psychological, or is there something wrong. Even though he's frustrating it would be hard to put him down.