Hate is such a strong word but...
I hate that my stubborn majano anemones keep coming back -- I really thought I had them under control, but they're starting to win again.
I also can't stand the algae outbreak my tank is going through ATM; I had to tear it down to catch a few fish, and the disturbance caused phosphates and nitrates to rise. PhosGuard is taking care of things, but for several weeks it's been a battle with simultaneous cyano, hair algae and film algae. Ugh.
Oh, and I HATE that my chaeto won't grow -- no matter what I do; additives, water changes, better lighting, pruning, begging... nothing makes a difference. It just breaks up into tiny pieces that get into everything, and refuses to grow.