Can you tell me why he may have died?

[pepperr]

Housing
What size is your tank? 5 gallons
What temperature is your tank? 78
Does your tank have a filter? yes
Does your tank have an air stone or other type of aeration? no
Is your tank heated? yes
What tank mates does your betta fish live with? no

Food
What type of food do you feed your betta fish? Hikari Betta bio-gold
How often do you feed your betta fish? 3 times a day

Maintenance
How often do you perform a water change? twice weekly
What percentage of the water do you change when you perform a water change? 45%
What type of additives do you add to the water when you perform a water change? Prime water conditioner, Nutrafin Cycle, Kent Pro-plant, and just started the last 2 water changes adding Reverse Osmosis water contidioner

Water Parameters:

I have been cycling his tank
Ammonia: 0.25ppm.... at 0.50ppm I do a 45% water change
Nitrite:0.0ppm
Nitrate:5.0 ppm
pH:6.0.... strange it is usually 6.8?
Hardness: n/a
Alkalinity: n/a

Symptoms and Treatment
So I had my betta for about a month now.
I fed him like usual and he seemed totally fine- Inquisitive, friendly, swimming peaceful and happy.
I left for the day and came home at night to find him floating at the top.
I figured he was having an episode of swim bladder, which he had once before.
I turned on his light to get a better look at him and he erratically swam the fastest I have ever seen him swim and was banging into everything such as the walls, floor, decor- as he swam his body was sideways and upside down. It was quite disturbing to watch but he only did this for about 20 seconds. Then he propped himself at the bottom corner of the tank in a kind of head stand position under a plant.
The next morning I woke up to check on him and he was hiding in his little home, dead...

How has your betta fish's appearance changed? It did not
How has your betta fish's behavior changed? He was floating before he died, and not swimming upright
When did you start noticing the symptoms? the night he died
Have you started treating your fish? If so, how? no
Does your fish have any history of being ill? he had swim bladder once
How old is your fish (approximately)?adult male, I only had him about a month

[Canuck Fins]

I'm sorry you lost your fish. It sounds like he'd lost his equilibrium if he was swimming upside down and ended up head standing. That does sound like he was sick, but I don't know with what. :(