Potential Problem, or Normal Fish Behavior?

[Frank15]

Two weeks ago, I had purchased ten harlequin rasboras for my 56-gallon tank, and all was going well. On that first day, they all stuck together super tight, but over the course of the next couple of weeks, were willing to spread further apart, and basically, all seemed happy and good.

Today, I introduced ten cherry barbs to the tank, and I'm wondering how safe it is for all of the fish. I was originally a little worried those cherry barbs might pick on the harlequins, but that's definitely not happening. Four of the ten harlequin rasboras have suddenly turned pretty angry, and seem to give angry chase to anything coming near them, whether it's a barb or another harlequin, including, oddly enough, the smallest of the ten. Another of the harlequins seems to have isolated himself in a large corner of the tank, no longer schooling with the other nine harlequins, caring only about making sure absolutely no one enters his large corner of the tank. Kinda' funny when the biggest of the four angry harlequins enters that guy's territory, at least!

So what is going on? The ten harlequins seemed to get along fine before the cherry barbs entered the picture. And now they actually seem busier picking on each other than putting up a united front before the cherry barbs, who seem perfectly oblivious to the harlequins when not being chased. Is this all normal for new fish being added, or something more serious here?

 

[chickenkev]

I have to say by far the most aggressive fish in my tank are my biggest 3 Harlequins, more aggressive than the angelfish, blue ram cichlid and red-tailed shark despite these being called 'semi-aggressive' and the Harlequins peaceful by most every website. I also introduced Harlequins first, and mine only really stick together when the lights are out. They all seem healthy and active though, as with the other fish so I assumed it was just a "get off my land" territorial thing, as they were there first.

 

[Mark Hewitt]

One thing you could do is to rearrange the tank, so they layout of ornaments etc is completely different.

I noticed in my tank, especially the black widow tetras would tend to stay in the same parts and not really venture out much. Then I rearranged everything and they're all over the tank now.