Here's my story and I hope someone can help me.  A couple of months ago, we decided to get a goldfish.  So I did the research on cycling the tank, etc.  We bought a 10 gal tank thinking we would get one goldfish.  I did fishless cycyling which took several weeks.  Once my ammonia & nitrites were zero and I had some nitrates, we bought a fish.  Nitrites started skying.  I kept doing water changes (about 20% a day), added Amquel+ every day and put in some aquarium salt.  The fish didn't make it.  So, I changed the water which brought my nitrites back down to zero.  After a couple of days, we bought another fish.  Nitrites went up, but not as high - they ran around .5ppm.  I put in the aquarium salt and amquel+, but just lost this one too.  Ammonia was always zero and ph was 8. 

Not sure what to do at this pont.   I'm assuming the nitrites are my problem.  Both fish looked fine, but started hanging around the bottom and were gone in a day.  Do I try cycling again?  Do I get another fish right away?  I really don't want to lose another fish.

The minimum tank size for a single fancy goldfish is 20 gallons. I would suggest buying a 30 gallon for a single fancy goldfish. Remember that these fish get pretty big 8" is about the average size that they reach in an aquarium. Your tank might not be able to handle the bio load that your fish is producing. Goldfish are very messy fish and the bacteria colonies that you have established might not be able to handle all the waste that your fish is producing. I would suggest watching your water parameters very closely and do a 50%-70% water change when they get to be hazardous.

Did you feed the fish a lot? Uneaten food will rot and create more ammonia, which can poison the fish. Try to feed in small pinches until the fish looks satisfied. There shouldn't be lots of uneaten food floating around the tank after you feed the fish.