I think we're forgetting about some very real health risks here. A lot of aquarium chemicals that we all use such as de-chlorinator, and various medications are toxic to humans. In fact, most of them have a health warning on the sides i.e. do not use for fish for human consumption.
There's also the fact that your fish died of natural causes, which says to me that something was very wrong (even if it was old age). Really, you'd be eating a fish who's organs had decayed to the point where they couldn't function any longer, so it wouldn't be very pleasant anyway. Plus the fact that aquarium fish don't get much exercise, and hence, would have quite poor muscle tone compared to wild fish, so the texture and flavour would be a bit off.
Bottom line, don't eat it, not for pleasure, nor for your own health.