Aggressive Platy Stressing Tank Mates – Need Advice

[sydneyroosters]

Hey everyone

Looking for some advice on managing aggression in my community tank.

Tank size: 28L (about 7.5 gallons)
Stock: 2 Platies, 2 Mollies (was 3), and 2 Swordtails.

The issue:
One of my Platies is extremely aggressive — constantly chasing the other fish, especially the Mollies. Even before I added four new fish this week, the behaviour was already there. Sadly, one of my 6-week-old Mollies died from the stress of being chased nonstop.

Now the aggression continues with the remaining fish. Not sure if it’s a male/female issue or just tank dynamics, but I’d really appreciate any advice on how to manage or stop this behaviour. Would isolating the Platy help? Or is this just something that happens in smaller tanks?
Video

Thanks in advance!

 

[Andy Sager]

For starters, you have the wrong fish in the wrong tank. Swordtails and Mollies get too big to be in a tank that small and you have the wrong numbers of fish if they were in the right sized tank. With any livebearer specie, you want to have at least a trio ( 1 male and 2 females) or more females to males even better. In a tank that small, Guppies or Endler's livebearers are better suited for it. They stay smaller, and with these species, keeping an all male tank will give you better coloring on the fish and you don't have the issue with overcrowding from all the babies they will produce if you had females. With Mollies, Platies and Swordtails, the males can be aggressive towards other males to have breeding rights to any females that are present. ( BTW, Platies and Swordtails can interbreed so the one male would need to be EITHER a swordtail OR a Platy. )
My suggestion would be to try to trade in these fish for other fish better suited for a smaller tank. Besides the Guppies or Endler's, you have fish like Chili Rasboras, Celestial Pearl Danios, Small Tetra species like Glo-lights or Neons or Embers. You could also do a single Betta but not with the other fish I mentioned as they can be territorial. For cleaning fish, pigmy cory cats can be used or use snails. The tank is too small for fish like Plecos or other algae eating catfish.
Until you decide or can trade in the fish, isolate the aggressive male but don't be surprised if another males starts in with the same behavior.

Hope this helps.

 

[sydneyroosters]

Thanks @Andy Sager so much for taking the time to write such a detailed reply — it was incredibly helpful! You’ve explained things so clearly, and it’s made me realise a few important mistakes I’ve made with stocking and tank size.

I had no idea about the male-to-female ratio with livebearers or that Platies and Swordtails could interbreed — that really puts things into perspective. I’m definitely going to look into trading in the current fish and restocking with species better suited to a 28L tank. The suggestions like Endlers, Guppies, and small schooling fish are super useful.

Appreciate your help — it’s made a big difference!

 

[Andy Sager]

Thanks @Andy Sager so much for taking the time to write such a detailed reply — it was incredibly helpful! You’ve explained things so clearly, and it’s made me realise a few important mistakes I’ve made with stocking and tank size.

I had no idea about the male-to-female ratio with livebearers or that Platies and Swordtails could interbreed — that really puts things into perspective. I’m definitely going to look into trading in the current fish and restocking with species better suited to a 28L tank. The suggestions like Endlers, Guppies, and small schooling fish are super useful.

Appreciate your help — it’s made a big difference!

No problem. It's why we are here.
It's the curse of the small tank. You think it's a lot of water but it's really not much more than a puddle of water to most fish. Now some fish enjoy a nice puddle and they can make a pretty tank but if you want really active fish, you need to do that in a larger tank. The longer and wider the better. Don't focus as much on the gallons ( liters) but on the length & width because fish swim left to right or right to left. They don't usually swim up and down.