Still need readings for ammonia, nitrite & nitrate, tank temperature as well. You may have a problem with the kribs being a little nippy, causing fin damage. A divider or another tank will help with this.
With any problems water changes can't hurt, and will help with any fin damage. If they are eating I would go this route first.
This is a good example of why a quar tank is a good idea, at this point it could be nipping, stress related, something they caught from your existing stock, or something they had when you bought them and just exposed your existing stock to.