I've been weaning my 2 cats off of a dry-food only (lower quality brand name) to a combination of higher quality dry food and wet food. I use the Chicken Soup for the Cat Lover's Soul brand.
I am doing it slowly. I am up to about 50/50 original food/new food. I have noticed that there is no solid waste in the litter box for a few days at a time. When there is, it is usually very compact and "hard" looking.
I do add some extra water to the canned food (recommended to do that b/c of one cat having urinary issues in the past).
Is this something I should be concerned about? Can I add any fiber to their food to make things move along better?

