Some heaters come with a plastic protection shield which surrounds the heater like the iQuatics Aquarad. They are easy to fit and help to prevent the fish from burning themselves on the heater. If the heater doesn’t come with a plastic shield then you can use a suitable plastic mesh but this may restrict the heating potential in your water.
The most effective way to totally prevent this from happening is to remove the heater and place it into the sump/filter area away from your livestock.