Well, Ive had my setup for about 4 years. My first fish, a puffer, I had for about 2 yrs. Im not sure it was my fault he died or not. He appeared to be totally healthy, then one day he just stopped eating for no apparent reason. No other signs of illness. I tried every thing, and every medication. He finally died after starving for about 5 months.
After that, I tried a shark egg. I sucessfully hatched him. He lived for a few months, but I couldnt get him to sucessfully feed and he died. Then I lost faith and let my husband maintain the tank. He had a humu humu trigger which lived maybe a year. Then some unknown fish, he bought cause it was pretty and didnt bother to find out what it was. He lived maybe 6 months. Most recently, he had a leaf lip soap fish, who lived about 6 months. Im positive that all of the fish that died in my husbands care, died from poor water quality as he was not on top of things like he should have been. (Also, along the way, we had things like blue damsels and crabs in there but those dont really count cause they mostly got eaten.) My tank is empty now, and I just redid it from scratch. Its cycling now, and Im hoping to get another puffer in there soon. (And Ill be taking care of him, not my "fish killer" husband!!)