Can Molly Tropical Fish be Aggressive?

Question: Platy And Molly Behavior

I recently rehoused 1 molly and 3 platys to a larger tank hoping it would lesson the aggression between several of them. The male platy is constantly chasing, sometimes nipping the red wag female. When the male platy is not chasing her, the black female molly is. When the male platy catches this he takes a thrashing out on the female molly (scraped sides, torn fins). Yet, everyone ignores the pregnant golden platy. She sits at the bottom of the tank by herself most of the time. She appears to have been pregnant for months. Is the aggression between platys common? Is the aggression between the female platy and female platy common? Is the aggression between the male platy and the female molly common? Is the pregnant golden's behavior normal or a pregnant platy or should I be concerned? This is all new to me.

Last edited by Bouv2; October 8th, 2010 at 12:34 PM.

Molly's and platy's are livebearers. Generally the males will harass the females if their are more males in the tank then females.

Looking at your current stock, I think some of the aggression is due to an overstocked tank. Molly's should be 1 in a 20 gallon, and platys do grow big. I think reducing the amount of your stock and separating males and females will help calm things down.

Oh my.........they had all been living in a 5 gallon tank and I thought the 20 would be ample. Neons and oto have not yet been rehoused to the larger tank and you say it's already overcrowded. .sigh.....

Oto catfish should be in groups of 3 or more
Cory Catfish should be in groups of 4 or more

Black Molly should be in a 20gallon or larger tank (can be aggressive)
neon tetras should be in a group of 6 or more
Use the 2F:1M ratio with platies. 2 Females for every male.

There should be no ammonia and no nitrite if your tank is fully cycled.

What are you using to check your ammonia, nitrite and nitrate levels? API freshwater master kit is what is recommended here.

I have a Master Kit and use it to test the water in both tanks.

The ammonia only shows in the little tank since it was overstocked, thus water changes 2-3 times a week.

Again, for the record, I unwilling inherited this group of fish who had lived the last 6 months together in a 5 gallon tank. I was trying to make their lives better by moving them into a 20 gallon tank. I quick learned enough about the water quality, cycling, planting, lighting and pairing to make it this far. Only intended to purchase the 2 otos and 3 neons to fill out their minimal numbers.

Attached Thumbnails Last edited by Bouv2; October 8th, 2010 at 05:54 PM.

A 20g is good for your current stock. My main concern is your molly. If you could upgrade to a 29g, you'd be able to add another male and female molly and that should alleviate the aggression and you'd be fully stocked.


True, what you need for the fish is-

by EventHorizon

Most fish need at least 1 or 2 gallons of water per inch of fish- so a 10 Gallon tank can hold 10 1 inch fish, or 5 2 inch fish... and that's only a guss-timate.
Then, if they aren't goldfish, you need a heater, and filter- best to get the best filter you can, it's worth it. Great filtration can keep your fish healthy. Diatom filters can actually filter out things like fungi and parasites!
Gravel is needed as well. About 2 5lb bags for every 10 gallons. Bags of gravel are about 3-4$. It's needed to colonize healthy bacteria that will keep the tank healthy.
Along with that, a syphon-gravel cleaner for water changes

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