I would have to say busted tank..just so much more that goes along with a busted tank besides the broken tank its self...
power outages..managed by generators
ich out break is a major pain in the back side but can be dealt with
heater sticking..i dont use heaters..my circulation pumps keep my water about 78 deg F
F & W raid..no issue here..i cant afford a fish they would be looking for any way
other..i cant think of anything worse than a busted tank followed by an ich out break since i have tangs
busted tank. worse experience i've had was when the furnace went out. lost everything ( i was'nt home) heaters could'nt keep up. almost gave it up after that.
Fish and Wildlife, enforcers of Laws that prohibit of restrict certain species of fish from being owned. so if in the U.S you keep Asian Aro's which have been outlaw since 1976 do to being endangered, than F&W could raid you house, take your fish, give you ten of thousands of dollars in fines or more, and possibly send you to jail.
by far its fear of a busted tank... the heater is no big deal cuz i check the temps all the time cuz im insane... as for power outages it would be a cause for concern but someone is always close to home and everything as antisyphon holes below the waterline this week im picking up inverters so i can run everything off car batteries if need be... and ich is no big deal if you pay attention to yer fish and notice it..