megapixels doesnt really matter, unless you are going for large prints which i doubt you will. lenses are more important than the camera (to some degree.)
just some basic terms:
aperture (f/) - a larger aperture (smaller number!) for example f/3.5 will minimize the depth of field, and because it is a large opening in the lense, it gives you a quicker shutter speed due to more light being allowed in. A small aperture (big number) like f/22 will give you a greater depth of field., but slower shutter speed
SO for macro shots, and lurring the background f.3.5 will be used most of the time, although you start to experiment once you get used to it.
ISO - camera's sensitivity too light. a low ISO (ISO100) is better, as it reduced noise (little dots on pictures) but it also gives you a slower shutter speed so not always ideal for taking fish pictures, unless you have a load of light over the subject, hence why people on here have studio lighting,
A high ISO (ISO1600) will give you a quicker shutter speed, but may induce noise on the image, but on better camera's they can handle higher ISO's better. I can remeber seeing one of George's shot with ISO 12800
Shutter speed - this is how fast the shutter closes. 1/125sec is quick enough for most fish. Trying to achieve this is a combination of the above.
I use aperture priority, This means you set the f/ and the ISO and the camera chooses a suitable shutter speed to get the correctly exposed image. eg i set it too f/3.5 and ISO400 and i get a shutter speed of about 1/60th sec. If i go any higher on the ISO i get considerable noise.
thanks.