
When you get diarrhoea (apart from dehydration not being something you'd want, young puppies especially) you can give a little natural yogurt (preferably goats yog. if you can source it) which normally clears up uncomplicated tummy upsets.
What you were doing at 2.5 weeks, was supplementing and should be seen as that until the end of week 3 going into week 4 when weaning would normally be started.
This response (looseness/diarrhoea) is an indication that their immature digestive systems are not ready to cope with external feed, solid or made into a porridge consistency. I'd have supplemented with warmed goats milk.
Our puppies, with weaning, had a porridge mix (after learning how to lap goats milk) plus lean mince, enough to cover a thumb nail, and only once a day, however much they love it! Again their immature dig.system.
I have no first hand experience of Leerburg formula.