@oatsbarley: Thanks.
Yeah, Slashee the Cow said it as well, the menus last too long. I agree, i shouldve just programmed a shortcut that skips the whole instructions and title screen when restarting.
As for the enemy-AI:
It is indeed very simple and running circles around enemies is just one way of outsmarting them. Another is to already align yourself properly on another layer, then drop down/up and just shoot everyone without any problems. However, i did not want to make the enemies frustrating by giving them the ability to shoot back or make them move too quickly. They are also usually, at least in my experience, dead before they really mass up in the middle. When you then switch to another layer they respawn completely randomly.
The differences between the dark fish are simply:
1: Low HP, normal speed, gives 1 point.
2. Very low HP, fast speed, gives 2 points.
3. High HP, slightly faster speed, gives 3 points.
4. Tons of HP, normal speed, gives 4 points.
Knowing that, it is wisest to just change between layer 3 and 4, but if one would struggle beating these enemy types, he could also stick with the easier types up top. That at least was the thought behind it.
Thanks for the detailed comment.