I believe your gun should be an 850 not an 830.
Make sure the low pressure selector is not on.
Use a single flopper spear, the double flopper tip is unnecessarily heavy. No need for a slip tip, it's for hunting big fish, something you wouldn't normally count on doing with such a small pneumatic.
Don't worry about the measure of how much air is in the gun. Don't take any air out. Pump it 50 times and see how it is to load. Keep adding 50 pumps until the gun is as hard to load as you'd want. I'm not sure but I think you can't over pump the gun with the factory pump it comes with. If you've reach the maximum pressure the little pump will give you and it's still not enough, you'll have to take it to someone who has a fitting that can feel the gun from a compressor or an air tank. In such a case they will have a gauge that will say how much air is in the gun.
This pneumatic should have no problem penetrating these fish completely from the distance you described.
There's a small possibility that on the fish you described the gun as bouncing off, the spear did pass through and hit a rock that was directly behind the fish.
Overall though the 850 is a small gun not meant to be used over any significant distance. The punching power it will have is for closeup shots only.