Is there something wrong with my betta?

[Cinclomin]

There's something wrong with my Betta but I don't know what it is. I got him about a month ago, he has a live plant and a few moss balls in his 2.6 gal tank. He used to make huge bubble nests everyday, and swim around. I put tap water conditioner, and a bio solution in and a bit of aquarium salt. A few days ago I was stupid and added a fish, he didn't stop chasing him so I re-homed the new fish. Now the Betta spends most of his time at the bottom and his fins are going clear and covered in black spots.
The fist picture is how he was when I bought him, the others are now. What's the problem? How can I help?

 

[Tropicalaqua]

Hello, i have had experience with betta such as the dragon fighter, i have found that they are renowned for sitting at the bottom.
this is often when they are week.
Bettas do not usually live for long, as they are mostly at their full potential in the shop.
the black spots can occur with age, however is most likely disease, you can treat him with drops from a local aquarium which you drip into the water.
try not to stress his or he could die.
hope that helped

 

[shellyx]

Hi, what's your parameters? I would keep giving him daily water changes at the moment.
Could he be changing color? Bettas do this with age.
70 gallon elephant nose tank.
30 gallon boraras , red longfin bristlenose, bamboo shrimp, crs,rcs and different snails.
7 gallon halfmoon betta and red ramshorn.