Fin Rot or Tail Biting?

[Litlover11]

I noticed today that Touchstone has a chunk missing from his fin. Is this from fin rot or tail biting? He has had fin rot before, but it never took big chunks of his tail off over night. If it is tail biting, what can I do to stop it?


Housing
What size is your tank? 5 gallon
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? None

Food
What type of food do you feed your betta fish? Betta pellets and the Occasional bloodworm.
How often do you feed your betta fish? Twice a day.

Maintenance
How often do you perform a water change? Once or twice every week
What percentage of the water do you change when you perform a water change? 25-50% depending on whether I clean the gravel.
What type of additives do you add to the water when you perform a water change? Conditioner and Stress Coat+

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

Ammonia:0
Nitrite: 0
Nitrate:10
pH: 8.5
Hardness: 25
Alkalinity: 300

Symptoms and Treatment
How has your betta fish's appearance changed? He suffered from what I thought was fin rot, but I am now suspecting may be tail biting.
How has your betta fish's behavior changed? It hasn't changed a lot, he is very active and has a healthy appetite. Note: He doesn't flare or build bubble nests.
When did you start noticing the symptoms? I noticed the fin rot a while ago after his filter stopped working. I noticed a large hunk of his caudal fin was gone over night when I finished giving him the four day treatment of triple sulfa.
Have you started treating your fish? If so, how? Triple Sulfa
Does your fish have any history of being ill? He had Fin rot last year when he was in his 3 gallon and he had what I think was a bacterial infection after there was a mini cycle in his 5 gallon in February.
How old is your fish (approximately)? 2.5-3 years old