Generally a proper diet and good water quality will help maintain a fish’s immune system. Sometimes it can be hard to properly assess the health issue of your fish, as there is quite a few disease’s they can get. If you find your fish are looking a little ill (not active or eating) then it is a good idea to test the water and keep an eye on them for any change in their appearance. If you can’t notice anything physically wrong with them, you can still give the pond a dose with a multi treatment or even some salt to give their immune system a boost.