Agressive Bosemani Rainbow

[mrhidley]

Hi, i have a 200litre community tank. I have 4 rainbows, 2 are Bosemani and I'm not entirely sure what the other two are, but i think they are banded. I'll put a picture in for identification. Over the last few days, one of the Bosemani's (which are juvenile) has become quite agressive towards one of the larger rainbows, he chases him around and nips him occasionally. Any ideas what's going on here? is it normal among Rainbows? or should i be concerned?
 

 
You can see the Bosemani i'm talking about below in the picture.

 

[N0body Of The Goat]

What are the tank dimensions? Feeling cramped for active fish could be part of the reason.
 
Keeping schooling fish in small numbers can make "peaceful" fish become quite aggressive.
 
Could all be kicking off now because they are maturing.

 

[mrhidley]

tank is 100cmx40cmx45cm
 
It's not a particularly serious issue at the moment, It's just the odd little nip and a  bit of chasing and normally the larger fish gives as much back as it gets, i'm just more concerned for when the bosemanis become fully grown...

 

[N0body Of The Goat]

The tank is really too small for these larger, active species of rainbowfish, 120cm long are often considered minimum but I think 150cm would be far better. It does not have to be wide or deep, 30cm would be fine. Your current tank could house the likes of Celebes; Threadfins; Dwarf Neon etc. for life.
 
In minimum groups of six, I believe these species tend to do better with two females per male. If you plan to keep these fish for live, I would gradually increase the number of both, but you really need to get a tank upgrade to entertain twelve 10-15cm SL rainbowfish.
 
What sex is the other bosemani? If it is female, the aggressive male could see the other rainbow as a rival and I believe the Melanotaenia species can cross breed to produce hybrids.
 
Not sure on the ID of the victim here, Melanotaenia herbertaxelrodi or nigrans sprang to mind, but I could be way off the mark.
 
On a sidenote, beware of housing fish like Neon Tetras with adult M. bosemani and alike, they have been known to be treated as live lunch.

 

[mrhidley]

Ah, i feel i might have been misinformed in the past, i was told that the rainbows would be peaceful with the tetras... Our long term plan is to re-house them in a longer tank and add more rainbows into the new tank, but it won't be for a few months yet. I'm not sure what sex any of the rainbows are as i don't know how to sex rainbows. Here are a couple of pictures of both Bosemanis.
 
http/s11.photobucket.com/user/corpse89/media/Fish/DSC02139.jpg.html
 
http/s11.photobucket.com/user/corpse89/media/Fish/DSC02138.jpg.html
 
If you could help, that'd be great!