Size matters. Even with koi. Are small ones strong enough? Or are the large ones lower maintenance? Is the immune system sufficiently developed in the small ones? Well, both have advantages and disadvantages! In any event it is clear that younger animals sometimes have a less stable immune system and are therefore more vulnerable to disease. It is also more difficult to treat them and the diagnosis is more complicated. The larger animals can have greater problem in adapting to a new environment and may fall more often ill as a result of this. Their period of acclimatisation is much longer than with the small ones!