Mollies Suddenly Being Aggressive

[bethan]

I've come for advice about my tank which was set up 6 months ago
it's 180 litres and everything was peaceful until the swordtails started growing up
one male began picking on another male about a week ago, I was advised by the pet shop to take the females out and keep the males (I had 2 females and 3 males)
the bullying didn't stop after I took the females out so I removed the more dominant male too.

the next day I got a betta and 2 lace gouramis to go with the remaining tank mates (5 neon tetras, 4 glowlight tetras, 4 platies, 2 female black mollies, and the last 2 younger male swordtails)
I just got in and the mollies have marks on their scales and bits missing from their tails!
I thought it was the betta who had done it as he was chasing them a bit so I took him out.
Then the mollies began fighting and were swimming really fast bumping each other and trying to bite each others side and tails.
I've now taken the most aggressive molly out and put the betta back in (I only have one small spare tank)

The mollies have been together for 4 months and have always got on well before this.
does anyone have any advice?
Thanks in advance

 

[wastegate]

You should really be keeping swordtail's at a ratio of 1 male to 2 females. I can kinda see your lfs's point in taking out the females to stop the males fighting over them but i would do it the other way around and remove the males, just leave one in.

Make sure your females are actually females. Some males wont develop their sword until a dominate male is removed. They will mimic been a female as not to atract aggression from the dominate male. To distinguish sex, check the anal fin. The males will be pointed and the females will be rounded.

I speak from experience. My first ever fish where 1 male and 2 female swordtails but it was only after the male jumped out of the tank and died that i seen what i thought was a female develop a sword (took maybe two weeks before it was noticeable). This fish is now soon to be a dad as the female is about to give birth any day now lol.

 

[BethK]

YOu haven't solved the aggression, you've taken out the females, only to replace them with another well known territorial fish.

Not sure why the mollies are going at each other but I personally think you need to take the betta out for now, sort the sword tail issue out and then let the community calm down before reintroducing the betta.

I would say that if a sword tail is a naturally aggressive fish, it may not be ideal to have with a betta. But I know nothing of swordtails x