Fish don’t really eat a high sugar diet, so I imagine not. Usually fish eat either proteins if they are carnivorous, or if they are herbivores they eat plants. When you eat or drink foods containing sugars, the digestive system breaks down the digestible compounds into sugar, which enters the blood. As your blood sugar levels increase, your pancreas produces the hormone insulin. Insulin prompts the cells in your body to absorb the sugar from the blood for energy or storage. This in turn reduced the levels of sugar in the bloodstream. Type 2 diabetes, occurs when the body can’t make enough insulin or can’t properly use the insulin it makes.
We don’t yet know if all fish produce insulin, this is a current topic of research.
I also think, if it is possible for a fish to be diabetic, which it may, they wouldn’t survive in the wild, because unlike humans they cannot have an insulin injection! Also, the process of natural selection would mean that conditions like diabetes would be very rare. This is an interesting area of research though, and hopefully soon we will understand the function of insulin in fish!