Help! My new zebra danios are ganging up and viciously attacking the other zebras. What do I do?

Question

I've had 6 long-finned zebra danios for quite a while now, originally they started in a 20 gallon tank, now they are in a 55 gallon with 6 cories. Last Monday I bought 4 zebra danios, to make a total of 10 in my 55 gallon. The new zebras have a yellowish shade to them, and I noticed after a few days that they were aggressive. Yesterday, I noticed 1 or 2 of the new danios going after one of my orignal danios. Today, I found 2 and sometimes 3 of the new zebras teaming up and attacking a single danio. Now before you say that this is their normal behavior, I know what their normal behavior is, and this is quite different. The group of new danios chase down and ram into the danio. It looks like they're going to kill one eventually. What is going on here? What do I do?

Answer 1

Take out the ones that are getting picked on into the old tank. Keep watch and make sure you get all the ones getting picked on!=)

Answer 2

weird perhaps feed them lol--joking

i think its like humans or maybe they want more space

r there a mixture of sexes?  if so they cld be competing for a particular female?

maybe they r competing for a leadership position

Answer 3

Any new, same species, fish will be subject to aggression by the "home team" to establish dominance.  And, notwithstanding your stated wisdom about ZD's, they are known for being agressive toward others; in this case you have some gang activity!

What to do? Warm your water by a few degrees. Be sure they are well fed (high protein - shrimp,worms,etc). Make sure your water chemistry and filtration is in order. Create some hiding places in your tank: a flower pot, short pipe, grassy plants, etc.  Good luck!