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Jul 23, 2018
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I have a 5 gallon tank. All fish were male.

Day 1: I placed 2 Mickey Mouse Platys and 1 Blue Platy in it.

Day 1 (6 hours later): 1 Mickey Mouse Platy is missing, assume he is dead.

Day 3: Introduce 1 Black Molly and 1 Dalmatian Molly with the 2 remaining Platys.

Day 32: Both Mollys are found dead within an hour of one another.

Day 34: Mickey Mouse Platy is dead.

Day 45: Slowly introduce Betta to tank with Platy.

Day 50: Betta is dead.

Tank numbers look good and the Blue Platy is still going strong.

Is my Blue Platy killing any other fish I add to the tank? Why?
What are your water parameters? Probably best to check this first although if they're all male it could account for some aggression.

5 gallons is a bit small though and I wouldn't have recommended keeping much more than 1 Betta in it so its possible you're just trying to put too many fish in too small a space.
No mention is made of whether this is a new tank or an established and cycled tank. But as mentioned above, a 5 gallon is way too small for all these fish. And ammonia and/or nitrite are likely the issue. Water parameters might be part of it too; we need to know the GH (general or total hardness) and pH of your source water (tap presumably).

Was the tank cycled, or is it new?

Welcome to TFF. :hi:
I also think it's most likely that the tank is not properly cycled and the fish succumbed to ammonia poisoning. Some fish (the surviving Platy) have a higher tolerance than others..... So is this a new tank? How was it cycled to develop beneficial bacteria (nitrosomonas / nitrospira [formerly thought to be nitrobacter])?.

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