Whether you are going to make them as fish pillows or going to use the 'fish' as appliques (to make a bigger pillow), I would suggest washing the fabric first, then starching the fabric so that is will cut and sew better. If you adding any other fabrics to the pillow, you should wash those, too.
The basic rule is either everything is washed or everything not washed. When you wash everything first the different rates of shrinking will all be the same when it is washed again.
Personally, I prefer to wash everything on a gentle cycle, a tiny bit of soap, and with a Shout Color Catcher in the wash. The color catcher will collect the over-dye and washing will get rid of any chemicals put in the fabric like formaldehyde.
Also, be sure to dry it in a hot dryer to get it to shrink the most.
Hope this helps.