Have you tried the regular guns that you load with bands. The benefit of those for people that may lack upper body strength (like me at certain times) is that you can regulate how much effort you need in order to load it.
It is also important to keep in mind that unless you have the perfect kill shot, you need to be able to handle the fish to dispatch it efficiently.
When you load a gun with soft bands (easier to load) the shot will be weaker and won't do a thing to a big/strong fish but will do fine to a smaller one. Using brand new bands takes a lot more force to load and in turn the spear takes off with greater velocity and well, you know, penetrates a big fat fish that may give you the fight of your life if your spear is tethered, or may go on badly injured to become fish food if your spear is free. Neither one of those options are pleasant.