My fantail fish died

Question
Hi, one of my fantails died recently and it looked like thier insides turned black along the front of it's stomach that was about 2 wks ago and just today another of my fantails died and the same thing was his stomach but not as much.  I had the fantails for a couple of years and still have 2 left plus blackmoores an a few other fish. I hadn't changed their diet except added blood worms but they'd been having them for months before they died. I have looked at other possible answers such as ammonia burns but my tank has been set up for years now, i thought it might of been old age but i believe they can live upto 15 years as i had had them from babies and they were about 3 yrs old.
Could you please help as i don't want an epedemic on my hands!!
VMT

Answer
Hi Rebecca,
I'm very sorry about your fish. Darker coloration in fish can indicate chemical imbalances in the water, parasites or bacterial infections. Even with an established tank it is always possible for a fish to suffer from Ammonia burns or Nitrate poisoning if the water quality is poor. Have you tested your water? If the ammonia or nitrates/nitrites are very high than you either have too many fish or don't have enough bacteria (ammonia) or you aren't doing enough water changes (nitrates/nitrites). All this can be solved pretty easily by either reducing the amount of fish in your tank, getting a larger tank, getting a better filtration system, or doing more frequent water changes. Though ammonia burns are pretty much permanent and there is no cure for them, you can stop it from happening again. And you are correct - goldfish live upwards of 12 years, sometimes longer than 20! But since so many people keep them in unsuitably small environments - such as bowls or even ten gallon tanks, they almost always die prematurely. The problem with goldfish is that they are large bodied, and therefore produce more waste and need more surface area in their tanks. If any of this applies to you than a few simple changes can save the life of your fish. And yes, goldfish can seem to be just fine for even years in a too small tank, but being exposed to excess amounts of ammonia and/or nitrites for a prolonged amount of time which will happen in a smaller tank can kill them suddenly one day with little or no warning. Best of luck to you.