Causes Of Fin Rot?

[-FighterFishh]

About a week ago, my male bettas fin rot seemed to be on the mend, but now he's started to get worse again.
The tank is the same, so is his diet, tank mates, decor in the tank, and he's acting normally in himself.

I'm just wondering what all the causes of fin rot actually are?
Maybe this way I can rule out what isn't causing it and try to find a way to cure it!
Thank you!

 

[Harlequins]

What are your readings?

Poor water quality/overcrowding/overfeeding/moving or handling fish can cause stress which may lead to fin rot...

 

[-FighterFishh]

What are your readings?

Poor water quality/overcrowding/overfeeding/moving or handling fish can cause stress which may lead to fin rot...

0 Ammonia
0 Nitrite
20 Nitrate

He's eating fine, hasn't ever been moved (except from when I brought him home, of course), and really doesn't act stressed. Haven't seen anyone nipping him, either.
Is there any other disease that rots away fins?

 

[CrazyAirborne]

how often do you do water changes and how much water per change? a lot of times I see lots of water changes prescribed as the best fix