I'm not sure about cats, but a hard tummy in dogs is a VERY bad sign. I agree with opus, I would seperate the cats so you can monitor the "sick kid" a little better. I would definately give the vet a call, better safe than waiting to long and having one less kitty.
Good Luck
Hi, I will try and help you as much as I can. first of all clean using vinegar, lysol or bleach and other chemicals are very hard on kitties...
meow mix and all dry foods are very high in carbs..and carbs cause weight gain and many other health problems...such as diabetes. don't free feed, stick to a schedule..2x per day.
If at all possible feed canned food and your kitties will be much healthier, but if it must be dry than use a grain free type. And be very sure they have lots and lots of fresh water at all times.
Has the kitty had a bowel movement? kinda hard to know when there are multiple ones in the home, try isolating him to watch his bowel habits, the hard tummy could be he's constipated. this is common on high carb foods, especially if he's not drinking enough water.
monitor that and see. otherwise not sure I can guess, only a Vet doing an examination would be able to tell you.
keep us updated♥