I have dead fish in my pond. Cold winters with low temperatures are often blamed for it, but most of the time that's not the reason. There's several reasons why your fish get sick and/or die. With new aquaria and ponds it's often the filter system that hasn't built up enough bacteria to get rid of the ammonia and nitrates. When there's a high population of fish it could be lack of oxygen so you need more bubbles.. But most of the time it's the sudden fall of of the pH levels in the water when temperature rises at spring. It starts around a water temp of 7 - 8 C or 45F. (and up). The pH scale measures how acidic or basic the water is and ranges from 0 to 14
. A neutral environment, 7, is appreciated by our fish, they can live at different readings though, but a sudden drop of the acidity could be disastrous. Below 4 you're in big trouble..
Many aquarists overlook the need of Calcium and Magnesium and the effect of a proper KH (Carbonate Hardness) in their freshwater aquarium/pond.
KH is basically the buffering capacity of your water, a KH above 6 helps prevent sudden drops in pH. (and is an important source of energy for nitrifying bacteria that eliminate ammonia and nitrite.)
It needs a test set to measure your water levels, you can get this at any petshop or garden center.
To have the KH increased you need to use KH+, ( available up to 5 kg bins). This probably will solve most of the problems already. It highers ph levels as well.
Another very important ingredient in a (Koi) pond is SALT. Not your Jojo kitchen salt, but special pond salt from the petshop/center. You need as much as 1g/ltr, yes that's 1kg to every cubic meter...( there's a salt meter to read on the levels.).
Once your pond not only has Goldfish or Shubunkin in it, but Koi and Sturgeons as well, it will change you, they become your friends for the rest of your life...