rainbow fish turned aggressive

[lanceuppercut]

I have a banded rainbow fish, a giant danio, as well as 6 other fish. they have all been in the tank for a couple years, and have been perfectly fine, until about a week ago. The rainbow fish has been chasing the danio around the tank non-stop. it looks pretty vicious, even though the danios fins are all fine and everything. The rainbow fish leaves all the other fish alone, it's just the danio.
What's the deal?

 

[cwt]

the giant danio probably looks the most like the rainbowfish, and since the rainbowfish has no other fish with which it can school up / socialize with, decided that the danio was close enough. You really need a few more rainbows to keep him happy.

 

[lanceuppercut]

Hmmm, maybe. It really doesn't seem like he's trying to "school" with him though. it looks more like he's trying to chase the other fish out of the tank! Unfortunately, I don't think I have room in my tank for three more rainbow fish,Its only a 30 gal. Will this have bad side effects on the danio, like over stressing him?

Also, why would this behavior start now? I've had both fish for over two years.

 

[cwt]

it might stress the danio. I have a single Siamese Algae eater in my 30G, which chases the other fish around; in another tank i have two, and they only chase each other. Why now? don't know. Either more room, or more friends, is probably going to be the answer.

 

[lanceuppercut]

Well, I have a 30 gal. planted tank with the rainbow fish, danio, 3 Odessa barbs, 2 red eye tetras and a Chinese algae eater. Do I have room for more rainbow fish in my tank?

 

[cwt]

the banded rainbow can grow up to 5", and the CAE even bigger than that, so maybe not.