Danio's being mean!

The zebra Danios are fin nippers. I have 6 zebra danios and I decided to add my betta. Five minutes later I see the betta trying to out swim the Danios. I had to take him out of there with the quickness. I think it has to do with the flowing fins. They don't nip at any other fish that I have.
 
I have three Blue Zebra Danios in my Community Tank, which at the moment has 20 fish. I have a 3/4 inch Clown Loach, a 2.5 inch Bengal Loach (or whatever they want to be called), three 1 Inch Dalmation Molly's, a 1 Inch Pl*co, 5 half inch Neons, 3 Beunos Aries Tetra's ranging from 1 inch to a monster 3 incher, 3 Head and Tail Light Tetra's, and my Blue Zebra Danio's.

In my tank I find that my Danio's literally have prizfights with each other and ignore EVERYTHING else. The Molly's and the Zebra's should as far as I know be the most aggressive fish I have, but my little Clown Loach tends to be the most aggressive fish I have.

I dont know if I am reapeating anyone here, and I am not an expert but this is from my experiance, I have had my Zebra's for about 3 months and had no problems with them. I think it may be your tank size?

Please correct me on any errors.

TY
 
I think it depend on the batch of zebra danios you buy at your LFS. I have a 46gal tank with 6 Zebra danios a they still end up messing with the Betta. Some are aggressive and some not.
 
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