Your tank has turned into a war zone. Fish with missing scales and tattered fins are hiding in the top corner of your tank, and there are three others that you have not seen in several days. What happened? My friend, you have an aggression problem. Never fear, you have come to the right place for information on how to solve this problem. You can win the battle; just follow these few easy steps.

First things first, sit down in front of your tank and watch what is going on very closely. Get yourself a cold beverage and have a seat. Spend some time with your fish, I mean isn’t this is why you bought them?

Do not get in too big of a hurry to “rescue” your abused fishes. The most important thing you can do right now is to decide which fishes are the aggressors, and which are the victims. Before you can fix the problem, you have to understand it. Is there one fish destroying everything else in the tank? Is it the reverse, is there one fish that all other fish in the tank is attacking? Is there a pair of cichlids… guarding a specific rock… covered all with eggs? You’ve got to figure it out.

If you have one fish that is getting slaughtered by everything else, then the solution is a simple one; move that fish to another tank, preferably a hospital tank, and treat him for his injuries. You do have a hospital tank don’t you? Every fish owner should have a spare tank set aside to treat and/or quarantine fish. It doesn’t have to be large, and you don’t even have to keep water in it most of the time, but have a hospital tank ready for emergencies, not to mention the fact that you should isolate all new fish for a couple of weeks before adding them to your established tank.

oscar cichlidSpawning fish are another challenge. When cichlids spawn, they’ll claim a vast percentage of your tank for their own use, and they’ll tag-team the other fish when they try to enter ‘their” now personal mating area. A mated pair of cichlids is a fearsome thing to behold, since they now fight as a team and a pair of cichlids is perfectly capable of driving off a fish twice their own size and killing anything smaller. If this is what is going on in your tank, you’ve got a couple of options. You can remove the other fish; remove the eggs; or install a divider within your tank. Doing nothing and hoping for the best rarely works, spawning cichlids have no pity.

If your problem is specific to one aggressive fish, you have a few options that might work. First off, include more ‘stuff’ in your tank. Piles of rocks, sunken ships, plants, almost anything will work. The idea is to create sight-line breaks for the fish, because with fish, “out of sight” really does mean “out of mind”. One aggressive fish terrorizing the others (in the absence of eggs) almost always means that the fish has claimed the entire tank as his own, and the best way to stop this is to box the fish in a bit. Oh sure, he’ll still swim around and boss the others around, but not as often, and not as violently if the others have a place to hide.

Another possibility is to actually add more fish to your tank. Now, you have to be careful to not overstock your tank, but more fish in your tank actually means more targets for the aggressive fish, and this means that each fish will take less damage as the bully won’t be able to focus on just one target.

The solution that is obviously the most fun is getting yourself a bigger tank, maybe that 225 gallon monster that you’ve had your eye on, but that’s kind of a long term solution as we don’t generally advise running down and buying a new tank on the spur of the moment. So if all else fails, it’s time to remove fish. Most people try to remove the smallest and weakest fish, but this is probably the worst way to go about it. Your best bet is to remove the dominant fish to another tank. Yes, you might have to get rid of it. If it’s your prize fish, well, then you might have to get rid of everything else. Then again, maybe it’s just time to buy that second tank. You have room for two tanks… right?