Coconut caves are great for any fish that likes to hide and that will fit in them. Kribensis. african dwarf frogs, african knife fish, and rainbow sharks love them. They may lower your pH too.
First grab yourself a coconut and drain it. I used a nail and hammer.
Try to drink it, get grossed out, and just drain it.
Score along the equator all around the coconut. Mine came pre-scored. I deepened it with a saw anyway.
Then take the blunt end of a cleaver and hit the equator until it cracks in half. Or a hammer and something flat.
Remove the coconut meat. I used a flat head screw driver, came out real easily.
Try to eat the meat, get grossed out, and just remove it.
The insides of the cocobut halves should be clean of meat.
Draw the doorway on the coconut.
Cut out the doorway using an electric saw or dremel tool.
Coconuts are chock-full of tannins. Boil it for 20 minutes, then replace the water... Then boil it some more. Keep boiling for 20 minutes and changing the water until it is nearly clear. You can leave the hairs on the coconuts, or take them off. If you choose to remove then, use a flat head screw driver and srape upward. You can also sand it if you would prefer a cleaner surface.
Rinse it off, and add some javamoss. You can also use it without the moss - it looks good either way.
Here is an example of the finished project.
Kribensis defending his cave