New Betta fish doesn't move!!

[Ashyxiv]

Hello everyone. Came up with an another issue I'm facing these days. I got a new Betta fish yesterday (Thai rosetail) from my local fish shop. When I bought him, he was in a very very little tank, I guess it's like 0.5 liter or something like that. Even at that time he was not moving (Actually not enough space to swim). After I brought him my home and moved into a 20 liter tank (same tank my previous Betta lived happily until his very tragic accident ) he doesn't moving much, Sometime he doesn't move at all. Because of this issue, I can't even feed him. My tank setup is pretty basic with black sand and three live plants plus a sponge filter. Speaking of this filter I replaced it today thinking the old filter is not working properly. But the same issue continuing. Yesterday he ate but today he doesn't eat anything. As the shop staff said he is below 6 months old. As I'm having other 3 tanks, I genuinely think there's nothing wrong with water or temperature. Any advice/idea/experience is much appreciated.

EDIT : I just checked his tank and noticed he constantly open and close his mouth as he chewing a gum.

 

[SaraB]

Not enough oxygen? Is he at least surfacing once in a while to gulp air?

 

[Ashyxiv]

I have no idea. [emoji3525] And yeah, I have seen him surfacing every once in a while. He looks so healthy but the behavior is complete opposite. Today I'm planning to put a dragon Stone in his tank. Hopefully he will enjoy swimming around it. Maybe is it because his new tank stressing him? Sometimes it's hard to read bettas behavior and it makes me stressed more than him. As always thanks much for your reply SaraB.

 

[SaraB]

Do you have live plants in there? It might help. An airstone might help too.
Hiding spots will help him feel more comfortable.

 

[Ashyxiv]

There are three natural plants in his tank. Good news is he's doing way better today, will see. Thanks much!!