Then I had a lightbulb moment, and thought "gilding flakes"!!!
Using a quickie glue pen I coloured in the fish again, and waited for it to go clear. Then I used some morris dancer gilding flakes from Indigo blu, buffed the image with my scootchie sponge and hey presto! A really lifelike fish!
The photo I took under the ottlite was the best one, as that really brought out the gleaming fish.
Otherwise, it was a straightforward card - 6" square base card, layered with some green textured card from my stash. I did some faux stitching around the edge and used punches for the circular sentiment (handwritten..) and a stamped Happy Birthday on some scrap card for the other one.