Had a surprise last weekend... One of the tanks I had filled with water and was cycling to get ready for some Angelfish suddenly had baby fish appear in it. I have no idea what kind they are or where they came from. I suspect they maybe baby Buenos Aires babies because I "seeded" the tank with water from a tank that had those in them. The eggs must have been in the gravel I vacuumed while transferring the water.
Then I had a snail appear in an established tank that has never had anything but the fish that I originally put in several months ago. That one is a real mystery to me.
Although I have nearly 20 different species of fish, I have been concentrating on Angelfish. Perhaps I should call my hobby "Immaculate Conception Fishery."