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Sep 6, 2019
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I have been having some trouble with platies in a 10 gallon tank. About four months ago I went out and got for platies just for fun because I was considering getting into fish keeping. At the time I did not know but I ended up discovering that two of the platies were male two of them were female and one of my two male platies (who I named Trey)became extremely aggressive towards my other male platy. It got to the point where I introduced two new female to the tank to try and increase my male to female ratio, but by then, Trey had nipped at the other males tail and fins to to point of death and blocked one of the female from the food so she starved. I isolated Trey while I cleaned the tank of the dead fish and tried to control an ammonia spike and lost the other two females. All I had left was Trey and one female that I named Minnie.

I was doing a water change and noticed fry in the tank and managed to collect 9 that are living happily in a smaller tank.

I waited a month, and introduced two new female platys to the tank a week ago. Trey immediately became aggressive towards them. Blocking from food and billing at tails. They coward in a corner and anytime they would move he would raise his fins and attack. I decided to give him up to a local aquarium store in my area and hoped my troubles were over. Now Minnie is turning on the two females. She nips and is blocking food as well. I’m at a loss and don’t know what to do. Any help or advise would be appreciated. I know I’m new to this and I’m maybe doing things wrong, but I have been diligently researching and trying to maintain my tanks to be healthy but I’m at my wits end with these platys. I thought they were non aggressive.


Supporting Member
Tank of the Month!
Feb 25, 2009
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The main issue here is the lack of sufficient space for these fish. Platies should be in at least a basic (high) 20g tank, but preferably (meaning, it is advisable) a long 20g or a 29g better yet. The 20g long and 29g have the same footprint (30 by 12 inches (75 by 30 cm) length and width, so the 29g would be better in the same space.

Platies are not exactly small fish, with males reaching 2 inches and females 3 inches, so they need space right from the start.

You were on the right track with the male/female ratio thinking, but there are always individual fish that do not adhere to the norm. If you can get a larger tank, I would suggest only males. One female will mean dozens of fry regularly, and not all will get eaten. And females acquired from the store may well be impregnated already, and even with no males present can still deliver several batches of fry.

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