I used to have about 20 wild caught halfbeaks in my 180 gal. tank. About 2-3 inch long fish. Dark grey in color.
When I stick my hand in the tank they all come and bite the hairs on my forearm. I could pull them out haning all over my hand. Creepy! Feels like leeches or something.
Also - we have about 70 tanks on racks and they are full of different kinds of fish. Some of them jump out and we find them in tanks that are not only under the "home" tank, but also way off to the side. The weirdest thing is to find a fish from a tank on the bottom in a tank above it. We tried to explain that with fish getting caught in the net and not being noticed but I'm not convinced in that.
Also - a tiny wild betta that came mixed with some other fish. Imagine a dull looking fish, grey in color, looking like a gouramy runt. Size - about 1/2 inch. This little thing single handedly killed about 200 Boraras brigittae. One by one. I couldn'd find a reason for the fish dying but one day I saw the betta slowly coming out, heading precisely to one of the dead bodies and biting it only once very hard on the side. Then in one straight line going to another body in another part of the tank and biting it once. Then in one straight line to another body in another part of the tank and doing the same thing. Looked like he knew exactly where they were and wanted to make sure they were dead. Didn't eat them. Creepy.
Syciopterus sp. gobies. These things can jump out of a tank, get crunchy dry, and re-hydrate when placed back in the tank. It seems that as long as their gills are wet the body can literally dry up crooked like dried fish.
Some fish (Syciopterus sp. for sure) seem to have some very strange sense for magnetic fields or something like that. They absolutely hate one tank - wriggle and jump all over like crazy. When placed in the tank next to it they imediately calm down and act completely normal.
Another strange thing - disease leaving only 1 fish alive. It has happened to us over and over again. From a batch of 200 fish all die because of visible or unknown problems. In about 7 out of 10 cases 1 or 2 fish stay alive. Not 3 or 4. Always 1 or 2. These 1-2 fish are completely cast iron. We still have a Chocolate gourami that was the only survivor out of 80. He heals from Ich faster than the Endlers that are in the same tank.
A funky looking kind of bottom dweller from S. America whose name I always forget. The fish is flat. Looks like a catfish that has been stepped on. This thing can lay in front of the tank without moving whatsoever for weeks on end. Literally 2-3 weeks in one place. Amano shrimp learn to walk over it. It doesn't get ticklish.
--Nikolay