Thanks for the super quick replies. I popped out and got some epsom salt. Mixed 1 teaspoon in a cup and poured it into his water. Pretty soon after he let out an enormous poo. He's still floating but I think that was the problem.
Anyway, here's the list, sorry I didn't fill it out. I thought I'd just drop the important parts into the actual post.
What size is your tank? 2.5 Gallons
What temperature is your tank? Thermometer is on the list of this to get. The fish shop guy said he doesn't use a heater with his and doesn't have problems. Middle of an Australian summer I guess helps
Does your tank have a filter? Yes
Does your tank have an air stone or other type of aeration? No
Is your tank heated? No
What tank mates does your betta fish live with? None
What type of food do you feed your betta fish? Packet says: Hikari Tropical Micro Pellets, great for tetras, barbs and small fish. They are about 0.5mm diameter dry, slighty bigger soaked.
How often do you feed your betta fish? Up to now 6 pellets twice a day. Will definitely be cutting back.
How often do you perform a water change? Had the tank for 6 days. Have performed 2 x 50% water changes. I'm sure it's posted hundreds of times around here, but what should I be doing?
What type of additives do you add to the water when you perform a water change? I use Seachem Prime in the new water that goes in. Says it removes chlorine, choramine and ammonia. Detoxifies nitrite and nitrate and provides slimecoat.
Fish shop guy said it wasn't worth getting a test kit.
Thanks again for the help, I'll let you know how he gets on. I'm fairly new to fish so all suggestions are welcome