My first ever retard fish was a blue glowfish that I named Ninja just because of this. He would hide so well in a tank with absolutely nothing blue in it. No one could find him. He scared the crap out of my grandparents doing this. He would spontaneously fly up to the front of the tank right where your face is and I kid you not he was saying "You can't find me! Hahaha!" and go right back into hiding where no one could find him. Ever. Four different people tried it. Never found him.
I had one guppy fry that I cleverly named Taz that would try to swim like the tasmanian devil in Looney Toons. He didn't last long.
My next one was my last betta. He was the most friendliest, docile betta I have ever seen. He was very curious about everything. He hated bright lights or objects, though. If something went into the tank or near him, he would slowly creep up to it to investigate it. Then if it moved, super ninja speed backwards away from it. And repeat. This is also the betta that was afraid of a snail. A nerite snail at that.
My current DG is rather odd. He thinks he's a gardener. He will adjust my plants, bite off dead leaves, keep them clean, and if he doesn't like where I stuck them, he will pull it up out of the sand and let it float until I move it. I have seen the evidence of him trying to replant some anacharis stems he trimmed. Just today he bit an anacharis stem in half and tried to stick it somewhere. He wasn't big enough.
My only "Nemo" fish is a cory cat named Lurch. I've noticed this week that his eyesight is fading. He was face planting straight into the lava rock that has been there for a good while. Today, he just hid behind the anacharis and had a panic attack when anything at all touched him. I felt so bad for the poor little guy. Although here just a little bit ago, the other two, Dracula and Lestat, have sort of taken him under their fin and are looking out for him. They figured out a way to tell him where to go so he can school with them. I've seen it. It's like they tap him when he needs to turn somewhere. I'm guessing he's seeing shadows, because he did freeze when my DG was right in front of him, but he still can't see very well, with freaking out if something touches him. He's a little more comfortable now that Dracula and Lestat figured out how to help him.
There is no such thing as an expert. Only more experienced than others.