Sick Betta!

I just purchased a new tank: http://www.petsmart.com/product/index.jsp?productId=2752385 because I wanted my betta to have more space to swim.

Unfortunately, after i purchased the tank (two days ago) it starting lying on the bottom of the tank. Is it possible the filter on the tank is too strong for him? I also added two tank mates when I bought the tank. A small bristle nose sucker fish and a snail. I've noticed the bristle nose produces a lot of waste but I read that the filter for the tank is really good, so I wasn't too worried about it. Should I be?

Here's the info from the sticky:

Housing
What size is your tank? 3 gallons
What temperature is your tank? 76 to 82 degrees
Does your tank have a filter? yes
Does your tank have an air stone or other type of aeration? no -- just the filter which aerates the tank
Is your tank heated? yes
What tank mates does your betta fish live with? snail and bristle nose sucker fish

Food
What type of food do you feed your betta fish? pellets and occasionally blood worms
How often do you feed your betta fish? every other day -- 3 or 4 pellets and occasionally a blood worm for a treat

Maintenance
How often do you perform a water change? Just bought the new tank two days ago
What percentage of the water do you change when you perform a water change? N/A
What type of additives do you add to the water when you perform a water change? A water equalizer

Water Parameters:
Have you tested your water? If so, what are the following parameters? Unknown

Ammonia:
Nitrite:
Nitrate:
pH:
Hardness:
Alkalinity:

Symptoms and Treatment
How has your betta fish's appearance changed? Color slightly muted
How has your betta fish's behavior changed? lying on its side on the bottom of the tank. However, it still is producing bubbles and has moments of swimming just fine. Eating is normal.
When did you start noticing the symptoms? Yesterday.
Have you started treating your fish? If so, how? No
Does your fish have any history of being ill? No
How old is your fish (approximately)? Purchased a few months ago

I would really appreciate your input! Thanks so much!!!