Inspired by "[another] M.C.Escher fish" and based on http://www.mcescher.com/gallery/symmetry/no-20-fish/, here's a 3D-printable model.
This is quite challenging to print accurately enough to tessellate multiple objects... many of mine needed some sanding or carving to make them fit. The bottom surface is chamfered so that any excess on the first layer won't interfere with the outline. I've shrunk the outline by 0.25mm to make it easier to fit together.
Anyone wanting to adapt this (reduce the tolerance from 0.25mm?) or see how it's done can get the model from OnShape.com -- search Public documents for "Escher Fish".
Slow speeds for accurate outlines. Don't over-extrude. Scaling this down in X/Y will make it more difficult to print.