NASCO sells a great product you can use over and over.
Forget the smell and mess.
They sell sets of reusable soft plastic fish,specifically made for this.
They are molded from the real thing, Carp, Bass, Trout, seahorses, starfish,
stingrays..they've got an impressive assortment.
I use printing ink and rice paper, but have also used Tempera and wrapping
The paper must be flexible , study , yet thin enough to mold to the details
of the fish. The white butcher roll paper works well too.
We rolled out banner size strips and did a watercolor blue and green wash
over it,let it dry , printed the fish and added seaweed to the bottom and got
some impressive results. Students ( all ages) think these are cool.