Second time is the charm...hopefully

So I decided to go for round 2 for trying a Betta in my 5g tank. I went to a different shop this time as I took the time to read the reviews on the first shop I got a Betta from. I went to that shop because it's where we always got fish from when I was a kid and it used to have a good reputation and it still does in terms of variety you can buy in both fish and all their related needs, unfortunately it no longer has a good reputation in the quality of the fish it stocks.
I looked around for another shop and found another not too far away but with much better reviews so popped across, had a chat with them about the first Betta, they felt it was probably an internal problem rather than a tank one but they did say if I bought one from them and the same thing happened then they would help me sort the problem out. The quality of their Bettas is much higher, they look more alive and vibrant. They're also double the size near enough of the other shops Bettas.
Gave it some thought then went back and got this chap. He's about a week in now, still alive and seems to be thriving, making bubble nests and harassing the shrimp (most of the ones still alive are big enough that the most he does is mock runs at them before pulling back). He's already clocked that the tank lid coming off means he gets a bit of food though not much because I don't want to overfeed him.
Hopefully I haven't jinxed him by posting about him. I've been a bit superstitious about it.

Nice fish. Good luck and I hope all goes well this time

If you have any problems with the fish, test the water quality for ammonia, nitrite, nitrate and pH, then post the results here asap, along with a picture. Do big daily water changes until the problem is identified or resolved.

The following link has information about what to do if your fish gets sick. It's long and boring but worth knowing.
https://www.fishforums.net/threads/what-to-do-if-your-fish-gets-sick.450268/