Which shrimp?!

[King puff]

I wouldn't put any more fish in the 40 litre because of the betta. Though it might seem fine if you do get something else, the other fish are stressed out, and your betta is stressed out which causes problems. Take ick for example, it is in almost every aquarium but only shows itself when fish are stressed. Since bettas dont need other fish, there is no point of putting other fish if you betta is happy on his own. The only thing that you could put with him and the Amanos is a nerite snail or another snail species.

 

[neoyyf]

I used this site to calculate stocking:

Aqadvisor.com

It is a great site, you shoukd check it out. I put in some numbers in for you and it states you are stocked to 101% which is ok if you dont add more. It also flags up that the Betta may become stressed living with shoals of fish, you must keep an eye on his behaviour or do what NickAu suggested:

That tank is perfect for the Betta and the shrimp, I would rehouse the Tetras

I think the tetras will do ok in your friend’s 46L

Make sure you are providing adequate filtration.

I would keep an eye on the amano shrimp, and see if everyone settles down. I would not suggest getting cherry shrimp as they will multiply.

I suggest planting the tank more heavily to get more hideaway spots for the fish and also to help with any excess nutrients