Green Terror vs Tiger Oscar

Hey everyone. First time posting here. I have kind of an urgent problem going on in my tank.

It's a 55 gallon tank with a rock cave and plenty of plant cover. I currently have:
one 7-8" tiger oscar
one 7-8" plecki
four green terrors, the biggest is about 4"

Recently the largest green terror has become extremely aggressive, especially towards my tiger oscar. This has been going on for about two days now. It wasn't a problem before, so why the sudden change? I'm really concerned for the oscar, as things are getting pretty violent (and even though the oscar is much bigger, he's on the receiving end).

Any advice?

Edit: also worth noting, these fish have shared the tank for over a year now. There haven't been any new introductions.

 

I would say that your tank is too small for your fish, which often induces aggression. Also, as fish get sexually mature, their aggression levels jump. Two of your Green Terrors might be starting to pair off, or thinking of it, making the aggressive one more territorial, which would account for the sudden change.
Keep in mind as well, that tho Green Terrors are great fish, they do tend to be more aggressive than Oscars.-------rick

 

You are running out of tank space. Green terrors and Oscars can grow up to 12 inches, and the 55 is barely adequate for just a single fish. Let alone having the Pleco in there, if it's a common can get towards 2 feet.

At around 4 inches, your green terrors are reaching sexual maturity, and one is trying to establish dominance in the tank. Regardless of them being different species, one will be the king of the tank.

So not much you can do as simply the tank is too small. You can try putting in more rocks and caves, but honestly it will only delay the inevitable.

Please do your fish the favor and start planning for larger tanks.

 

The fish that you currently have in the tank will get far to big for it. For one Oscar/Green Terror alone I would recommend a 75G minimum. With all those Cichlids in that one tank, it's not surprising that there is some aggression. Like ricmcc said, as the Cichlids begin to mature and grow, they will need more territory to claim. I would advise to rehome and/or upgrade to prevent this from happening.

 

Welcome to FishLore! I hope you enjoy the site.

 

Recently in the same boat... after a move & some water quality issues, my 55gal was only inhabited by an almost 6" yellow lab cichlid, who I've had for almost 7 years... I saw a beautiful 2" green terror at my local fish store and just had to have him. I bought him and two others. Ended up with two males and one female. Within a month, I had to find a new home for the smaller male G.T. and a new tank for the yellow lab. He also bullied the female some, but once it was just the two of them in the tank that stopped...

Boris is now almost 4" and Natasha is almost 3". I'm hoping they'll breed (he's displayed spawning behavior) but they're still kind of young. I've had Oscars before and loved them, but I gotta say, only a few months in and I'm thinking I the Green Terror is my new favorite fish. She hides a lot and he will herd her to her favorite 'cave' whenever I come into the room. He is always watching me, even when he 'hides' - I can see him peeking out. He reacts to any movement big or small, and will 'play' with my finger on the glass. He even plays with my cat, who would always sit and stare at the yellow lab, but not 'play' with him......

Maybe, if all goes well, I'll realize my dream and get a 100+ gal to have a colony tank. Maybe Boris & Natasha will spawn and make that happen..? Got my finger crossed!