An ice cap over your pond is not a great thing. It prevents gas exchange between the air and your water. Gas exchange is important for getting oxygen into the water, carbon dioxide out, and for dispersing the pollutants that build up in the water. Insufficient gas exchange can stress and even kill your fish. Plus, you can’t generally see them through thick ice.
However, when your water does ice over, you definitely don’t want to break it by hitting the ice. Here’s why:
Water is extremely resistant to mechanical compression. When you push on water, it doesn’t squeeze down, but moves and transmits that force to somewhere else, pushing against whatever is containing it. This is why hydrolics work, but it also means that a hard strike to the ice on top of the water gets transmitted to everything else in the water, including your fish. In the wrong circumstances, hitting a solid ice cap on your pond can be like directly hitting your fish. This can injure or kill them.
That’s worth saying again: hitting the ice on your pond to break it can kill your fish.
The best way to deal with an ice cap on your pond is to prevent it from happening, using either an aerator or a surface heater. If the cap forms before you can prevent it, use a drill or saw to get through it, just like you’re ice fishing, or use a surface heater to gently break it up.