What is the size of your tank?
Is the tank and the fish new?
Your fish is small right now, but keep in mind they tend to outgrow regular goldfish bowls.
Also, tanks should be cycled first, in order to get some "good bacteria" in your tank. If the tank isn't cycled, it can kill some fish.
Also, do you have a heater? Goldfish don't really like heaters.
Yet again, it could be nothing. My fish go to the bottom of the tank all the time. It's just better to be safe than sorry.
What size is your tank?
What temperature is your tank?
Does your tank have a filter?
Does your tank have an air stone or other type of aeration?
Is your tank heated?
What tank mates does your betta fish live with?
What type of food do you feed your betta fish?
How often do you feed your betta fish?
How often do you perform a water change?
What percentage of the water do you change when you perform a water change?
What type of additives do you add to the water when you perform a water change?
Have you tested your water? If so, what are the following parameters?
Symptoms and Treatment
How has your betta fish's appearance changed?
How has your betta fish's behavior changed?
When did you start noticing the symptoms?
Have you started treating your fish? If so, how?
Does your fish have any history of being ill?
How old is your fish (approximately)?
If your tank is small than 20 gallons, and if you do not have at least one filter In the tank with water changes several times a week, you don't have a long term set up. Most goldfish grow a good 7 inches to a foot, and live 10 or so years. In general, they are super messy fish and poop like nobody's business, so improper care of the tank could lead to a up build of ammonia, which can seriously hurt your fish. Healthy goldfish don't lie still, they are typically active.
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.
+1. If you can't afford a 20 gallon tank, I suggest getting a 20 gallon storage tub instead and spending what you can on a filter rated for 40 gallons with a turnover of 200 gallon per hour. He'll start to double in size every few months and eventually turn into a 6-10 inch monster. Storage tubs make fantastic temporary fishtanks.
If you can find 30 gallons, though, I'd suggest that. Our comet (currently in a 23 gallon, being moved to a pond in a month) is ten inches long and looks cramped in his tank. If you can find 40 gallons, even better - then you can get your fish a friend. Goldies are social creatures, so if you do manage to get enough water volume, it's a good idea.