deteriorating fins

[ggriffin411]

I'm not sure what is wrong with my fish. When I got him in September his tail smooth at the edges but thin. I keep him at school so he is alone all weekend. On a Monday morning in October I noticed his tail fin was shredded. I thought he might have been tailbiting so I rearranged the tank. The fins looked melted at the edges but not black so I read up on fin rot. Most posts recommend clean water as a preventative and cure. According to my LFS the water is perfect. One post mentioned lack of minerals (I use city water conditioned with Tetra Betta Safe) so I started adding a "Wonder Shell" to the tank. My fish is very active and eats well but his fins are still deteriorating. I'm not sure what else to try.

Housing
What size is your tank? 2.5 gal with silk plants and marbles on the bottom
What temperature is your tank? 80
Does your tank have a filter? sponge filter
Does your tank have an air stone or other type of aeration? yes
Is your tank heated? yes
What tank mates does your betta fish live with? no other fish

Food
What type of food do you feed your betta fish? Betta pellets & dried blood worms soaked in tank water
How often do you feed your betta fish? 3-4 pellets a day - worms for a treat sometimes - no food on weekends

Maintenance
How often do you perform a water change? 50% change 3 time a week

What type of additives do you add to the water when you perform a water change? 1/2 tsp aquarium salt Tetra Betta safe water conditioner

Water Parameters:
Have you tested your water? LFS tested water and deemed it "perfect"

Symptoms and Treatment
How has your betta fish's appearance changed? tail fin appears to be melting and now pin holes have appeared in dorsal fin
How has your betta fish's behavior changed? no, he is very active
When did you start noticing the symptoms? 3 weeks age
Have you started treating your fish? If so, how? did 50% water changes every day for a week
Does your fish have any history of being ill? new fish in September
How old is your fish (approximately)? ????

[Oldfishlady]

Can you post a pic and list all the test result of both the tank water and the source water itself with and without additives for-Ammonia, nitrite, nitrate, pH and if you can get the GH/KH too.

What tail type is he, how much water movement does the sponge filter cause.
Has he been kept in the aquarium salt since you got him.
Anyway you can improve his nutrition
Sometimes, if they are not biting their fins-water movement can cause fin damage, however, usually sponge filters are pretty gentle and don't cause fin issues-unless they cause a lot of water movement in a small tank.
Marble and other items that can pinch the fins can cause damage

Post a pic and that will help us tell more about what the cause might be....