Fish Being Eaten!?

[vanimal]

My brother has a 20 gallon tank. This was his stock
1 Dwarf Gourami
5 Neon Tetra
3 Long Finned Danios
3 Emerald Cories
2 Mosquito Fish
For 2 months all the fish were doing fine until he added a Dwarf Gourami, and 3 Cories. Since then fish are having their tails eaten off. We assumed it was the Gourami and we moved him to my 150. I saw his tank today and now he's down to 2 Danios, and 3 Neons.(all with their tails/fins nibbled) It only got worse. The only fish that aren't hurt are the Cories and the Mosquito Fish. I assumed it was the Cories because the Mosquitos were in there for a long time with no problems. What do you guys think???
Thanks

 

[Aquatic_Adam]

My brother has a 20 gallon tank. This was his stock
1 Dwarf Gourami
5 Neon Tetra
3 Long Finned Danios
3 Emerald Cories
2 Mosquito Fish
For 2 months all the fish were doing fine until he added a Dwarf Gourami, and 3 Cories. Since then fish are having their tails eaten off. We assumed it was the Gourami and we moved him to my 150. I saw his tank today and now he's down to 2 Danios, and 3 Neons.(all with their tails/fins nibbled) It only got worse. The only fish that aren't hurt are the Cories and the Mosquito Fish. I assumed it was the Cories because the Mosquitos were in there for a long time with no problems. What do you guys think???
Thanks

Corydoras wouldn't do that.

An unseen culprit could be fin rot. Adding the DG and the Cory's could have upset the parameters of the 20g.

Either that or since your friend doesn't have a lot of Danios, they started nipping everyone?

 

[vanimal]

Well they're both nipped to and they were fine until I added the new fish. Wouldn't fin rot affect the Cories too?

 

[Aquatic_Adam]

Time may tell when they do have fin rot. They are really hardy fish do they could just take longer to be effected.

 

[vanimal]

Ok thanks.