Aggressive Molly

[rubiix]

Two days ago I bought a pair of dalmation mollies to add to my tank. I believe I bought a male and a female as one has a pointy anal fin, and the other seems to have two much more rounded fins in the same area. Until yesterday (the next day after purchasing them) I hadn't had a chance to see how they were settling in. It was then I noticed that the male molly seemed to be periodically chasing one of my balloon mollies that was already in the tank, trying to nudge the bum area of the fish. It has also been chasing the other balloon molly in the tank a little, while the other dalmation Molly has been largely ignored. It seemed to be quite aggressive. I wasn't able to be home to monitor the problem, so I placed him in a spare breeding net overnight. I have let him back out today and he seems much more subdued and seems to be sticking to one corner and leaves the other fish alone for now.

Could this be a case of him trying to establish some territory, or establishing dominance? Will the problem settle down by itself? Should I get him another female and if so does it need to be another dalmation Molly? I'm still in the process of cycling another tank twice the size so he will in a few weeks have some more room.

 

[the_lock_man]

I am assuming that the two balloon mollies are females? Balloon mollies are not genetically different to dalmation mollies, they have been selectively bred to produce that mutation.

Assuming they are females, he's just trying to reproduce. Male livebearers have three thoughts. (1) Can I have sex with it? (2) Can I eat it? (3) Can I have sex with it again? I suspect the reason why he was subdued is because of the stress of being kept in the breeding net. Once he settles down again, he'll go back to chasing the females, including the female dalmation. It isn't problem, it's his natural behaviour, and putting him on the naughty step (ie the breeding net) is just going to stress him unnecessarily. It sounds like he has plenty of females to chase, so the females won't get overly-stressed.

On the other hand, if the balloon molly he was chasing was a male, then yes, it's a dominance thing. Again, not something you can control, except by removing him completely.

In general terms, you need to have a ratio of 1male to 2 or 3 females with any poecilid livebearer species.