Why is this fish so aggressive?

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Max Pedley helps a reader understand the reasons his fish are being bullied in the tank.

Q: I recently bought a trio of Apistogramma cacatuoides for a 60x35x35cm set-up, with a small shoal of Gold tetras. There were several caves, places to hide and the tank was well-planted. However, one of the two females was being badly bullied by the other female so I moved them into a larger 90cm long tank, again with plenty of hiding places and plants. But even then the same female sought out the second fish and bullied her until she just spent the whole time cowering behind the heater. I tried removing the fish and re-arranging the decor but it didn’t help. The spare female is now separated and back in the smaller tank, a bit tatty, but okay. However, now the other female has started berating the male and I’m contemplating moving him too. Is this normal behaviour for these fish? I thought that apistos did well in trios, or do some species work better than others? Or do I just have a very stroppy female?

JEANETTE CROSBY, VIA EMAIL

A: Max says: In nature, most Apistogramma are indeed polygamous fish, often with one male to many females. As such, you will no doubt find a great deal of aquarium literature which will advise keeping Apistogramma in trios. 

Unfortunately, with limited space in an aquarium setting, harems are usually impractical, often resulting in female on female aggression, as they jostle for territory within that of the male.

In larger tanks of 150cm upwards, a group may be kept, but one should still be conscious that disputes between females do occur. Occasionally, males may even show such a strong paternal instinct that they will faithfully pair with one female and attack other females within the aquarium.

In your scenario, the female now bullying the male could be a sign that she has eggs or even fry. Such harassment is common where the females do not feel safe rearing fry when the male is in the vicinity. 

It might be worth watching to see if she has lost significant mass around her abdomen or is showing bright yellow spawning colours.

I think it would be wise to stick a single pair in your 90cm aquarium from now on.