My biggest fear has already happened t me before: mass fish dieout. Only the mollies, 1 GBR, and a mystery snail survived, I lost a German Blue Ram, 9 Black Skirt Tetras, and a mystery snail.
Right now however, my biggest frustration is that all my tanks are on the floor. I want to put them where I can view them easily, but to do that I need to break them down and then rescape (never get the same scape twice) and I love the current scape. It's a pain to choose between a scape you love and being able to see the tank without lying on the floor...