Of the old pets, 3rd was a Shrimp (bout one inch long), 4th was a Crayfish (started at an inch, died at about 5inches.. when one of the fish ate it).
Shrimp was pretty smart, I used to feed the fish when I got home from work, one week I was off work and noticed that still at that same time each evening it would come to the front of the tank and sit in wait for the food. The Crayfish used to move rocks/gravel/sand around the tank to build a cave. Would barricade itself in to shed it's shell, until about the 4th time it did it when it was quite a bit bigger and apparently didn't bother to hide... the Oscar I had at the time ripped it in half >.>