Join Date: Oct 2010
Location: Victoria, Australia
Unfortunately, 2.5 gallons is way too small for a goldfish. You are going to eventually see problems resulting from poor water quality and stunting.
You really need a 20-30 gallon tank for a single fancy goldfish as although they do not grow as long as commons, they can grow to the width of a baseball through the body.
Your filter should be turning over 10 times the volume of your tank every hour. And you need a proper filter as this provides a surface area for beneficial bacteria to colonise, and with the amount of waste a goldfish can produce that is going to quickly become an issue.
Goldfish also require a large amount of dissolved oxygen in their water, which is why they fail to thrive in bowls with no source of surface agitation or flow.
I highly doubt your tank is cycled. It is difficult to cycle anything under 5 gallons and hold parameters stable, and without a filter, it is essentially impossible.
I would advise purchasing a minimum 20 gallon tank, a filter that is capable of turning over your tank volume times ten in an hour, and test kits for ammonia, nitrite and nitrate as you will soon be in the middle of a fish-in cycle.
Goldfish are supposed to live longer than the average dog or cat. However, being carp, they are hardy enough to live in less than ideal conditions for quite some time. Please do not confuse the fact that your fish is visibly 'fine' with the notion that it is healthy and thriving.
This is why the myths about a goldfish in a bowl persist, as most people assume that a goldfish is only supposed to live for 1-2 years, and grow the size of its tank.