aggressive mollies and guppies

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Jharvey

Aquarium Advice Regular
Joined
Jan 11, 2016
Messages
57
Location
New England (USA)
Hey all - I feel like the fish in my community tank are becoming more aggressive. A couple of weeks ago I noticed my smaller guppy hiding in the tank and found that his back tail fin was starting to look shredded along the edge (but no spots or discoloration). I thought my biggest molly was to blame because she often chases any/all of the fish but I've never seen her nip. I put the guppy in a 5-gal QT with 1T of aquarium salt and he seemed much better within a day. He was swimming around the QT freely. I couldn't tell if his tail was getting better, but it wasn't getting worse.

Today I noticed that my second guppy's tail (he was still in the community tank) was starting to have the same shredded look (again, no spots or discoloration). I decided to move him into the QT with the smaller guppy. When I got home about 4 hours later, the smaller guppy was hiding in the QT and when I moved the decoration where he was, saw the bigger guppy definitely nip the smaller guppy's tail fin. Ahh! So, had to move the bigger guy back in the community tank - not sure who might be nibbling on him but he doesn't seem at all stressed when he's in there.


Is something about my tank or habits making my fish aggressive? Or are they like chickens who pick on weak-seeming peers? Is it problematic to have just two male guppies? Could the fin shredding be from something else? Or....?


specs:
I have a 29g tall tank established for 6 months. Ammonia and nitrites are at 0. nitrates are between 5-10ppm (I have such a hard time reading that one), 81 degrees; pH 7.8

I have an aquaclear 50 HOB filter and a tetrawhisper 10i that I keep in the main tank in case I need to instacycle a quarantine tank.

I have 2 guppies (male); 3 mollies (female); 6 neon tetras; 4 cardinal tetras;

I do weekly water changes/vaccumings of at least 5 gallons. (Did a 15-gallon WC two days ago).

I feed them Aqueon tropical flake food once per day.

I haven't added any new materials, plants, or in the last month. (But have aquarium gravel for substrate, a couple of aquarium decorations, 4 real plants, 2 fake plants)

Sorry, I can't manage to get a clear picture.
 
You need a bigger tank if you want to have a community tank with mollies. The agressor's are the mollies, not guppies. Female or male dosent matter, mollies are agressive and terrirtorial, and will nip and bully bite other fish.
 
I have never heard of mollies being aggressive or territorial before - I certainly can see it in action now. Unfortunately.

When my guppies were together, one was definitely nipping the other. Have you seen that between male guppies?
 
Yes, they will fight to establish a hierarchy as will the mollies. Mollies being much much more agressive. Guppy fights between males or females are usually pretty harmless. Both are poecillia family.
 
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