For freshwater fish as Koi you may be able to get by without having one. Freshwater fish are generally more suited to captivity because they are usually tank raised and don't seem to break out in disease as readily as their saltwater counterparts. However, if newly acquired fish do come down with something, you will surely wish that you had one ready to go. One newly bought Koi fish that is introduced to your main tank can easily wipe out the entire tank population. Better safe than sorry, right?
