My two Oscars get along great. Today I bought two new fish. A silver dollar and a firemouth. The silver dollar is about the same size as my Oscars and the firemouth is about 20% larger. Now one of the oscars is kind of being mean and chasing the firemouth and silver dollar around, if they get close to him. Is this only because the two fish our new to the tank and the Oscar wants to establish his dominance, and will stop... Or is his behavior most likely going to continue?

if the Oscar was there first then this is probably why this is happening.not really sure about pecking order, more about just being a bully. We got to remember that fish can't really get away from each other in an aquarium. In the wild, they were probably just avoid each other. If you've got semi-aggressive fish with very placid fish, one of them is going to except the bullying without any fightback. Silver dollars are extremely placid, I can't imagine them bullying on Oscar or fighting back

As for carrying on, could well do, or they might just calm down.

It just seems so odd to me because the firemouth is larger than the Oscar and it just runs away.

Nobody can win against oscars! they r the boss!

anyways, my oscar sometimes chases my pleco, but most of the time they r friends, i think it depends on the Oscar's mood, they become aggressive when they're hungry or when the water is too warm.

Jeffrey wrote:


But my Oscars are ALWAYS hungry!

Firemouths generally will show some aggression but when shown it back they tend to back down. The dominant fish isnt always the biggest. Some oscars will tolerate new fish in the tank, some wont, all fish are different. Hopefully your oscar will get used to the new fish and tolerate them in the tank. If the new fish get injured or spend all the time hiding away then it is time to think about either rehoming them or the oscar

There are many reason for a fish becoming aggressive, some are temperature too hot, not enough food (like not eating for a week- oscars always look hungry) or overcrowding, or as you are finding out new fish added

Keep an eye out for aggression between your oscars as new fish in the tank can unsettle a tank and they may feel a need to re establish a pecking order amongst themselves

Thanks, It has already seemed to settle down. The Oscar still shows its dominance every so often but it is not really chasing the firemouth anymore.

But the Silverdollar is just afraid of everything. Would I be better off if I got two more silver dollars so they are in a school instead of just alone?

Silver dollars are best kept in groups but they can live alone. Your SD should soon settle in and realise he will be ok in the tank. If he is still hiding away all the time after a week then think about rehoming it or getting more. The problem with getting more is that your oscar might do to them what he did to your SD, the advantage of getting more is any aggression is spread around so 1 isnt bullied all the time. Oscar dont usually hold grudges so he wont hassle the SD forever


Do you have enough room for 2 oscars, a firemouth and 3 or 4 SD's. I would say you would need at least a 100g tank

I have a 125. I am just slowing sticking it.