aggressive saltwater fish

Dealing with aggression between fish in a marine aquarium

Albert B Ulrich III Saltwater Fish 2 Comments

Aggression Between Fish in a Marine Aquarium

Unfortunately, aggression between fish in a marine aquarium is a common problem. Let’s face it, a coral reef can be a tough place to make a living, and the fish we keep in our homes have evolved to survive in that fish-eat-fish world. When we pluck them off the reef and place them in their new home, it is only natural to expect a few scuffles along the way.

aggression in marine fish

The Yellow Tang can be an aggressive marine fish–image by Clevergrrl

However, when scuffles turn into major aggression issues, serious problems can ensue. Harassed fish may become injured, sick or even die.

When it comes to aggression between marine fish, like many things in life, an ounce of prevention is worth a pound of cure–or if you prefer the metric system–28.3 grams of prevention is worth 0.45 kilograms of cure.

I’m not sure if those decimal places are significant figures…

When planning ahead, to prevent aggression between fish in a marine aquarium, the best thing to do is plan to add the fish in order of aggression–from the least aggressive first, to the most aggressive last. The premise behind this advice is as follows:

  • Most aggression between fish is territorial in nature–Fish A is defending their territory, feeding place or resting place from Fish B.

  • When you add the fish in reverse order, you allow the less aggressive fish to establish their territory before a more aggressive fish.

If you follow that protocol when adding fish to your marine aquarium, the more aggressive fish, Fish B in our scenario, is actually being added to the more meek fish’s territory, and the more aggressive fish doesn’t yet even have a piece of turf to battle over.

Of course, this little strategy is not a cure for mixing incompatible species, but it is certainly a great place to start.

Here are a few other tips for dealing with aggression between fish in a marine aquarium:

  • Don’t mix fish from the same family, stick with the one-fish rule–one fish per family, per aquarium

  • Don’t mix fish that look similar to each other–mixing fish like the royal gramma and royal dottyback is a recipe for disaster

royal gramma marine fish

Royal Gramma Image by ahisget

aggression between fish like the royal dottyback and royal gramma is common

Royal Dottyback image by Psym


  • Don’t mix fish that occupy the same niche in the tank–if you mix niches, the fish may fight with each other for their own turf

  • Don’t add fish that are notoriously aggressive–if they have a reputation for being aggressive, leave them at your local fish store–damselfish and maroon clownfish are notoriously aggressive fish–don’t expect them to be mild-mannered in your tank

  • Keep your fish well fed–squabbles often happen between fish over food or territory. You can’t give them more territory, but you can always give them more food. Keep them well fed so they don’t need to fight over food.

You probably won’t be able to completely avoid aggression between fish n your marine aquarium–fish will chase each other, nip at each other and even try to impale each other (have you ever seen a tang threaten another fish with its tail spine?), but by following the tips above, you will be ahead of the game in managing the aggression.

Written by Al Ulrich

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Albert B Ulrich IIIDealing with aggression between fish in a marine aquarium

Comments 2

  1. Jon

    Good article but I do believe that feeding causes issues. I have seen many fish that are fat and aggressive but I have never seen a fish that is thin and aggressive. They are too busy trying to forage. If they are all foraging they have no time to fight. If you had to farm and hunt for your food would you cooperate more with your neighbor or fight about the fence he put in? Or how long his garbage can is out? etc.

    Also, wouldn’t mature mating fish be more aggressive? Fish can’t attempt to procreate or give their spawn the best chance of survival if their territory isn’t secure. Not that they care for offspring as a croc or we do but they do have hormones that kick in when they become sexually viable. If I am correct, most fish spawn during the summer which is from excess food available from corals producing sugars. This causes spawning because nutritionally they can handle it. Now if you drop these highly vibrant fish in your aquarium or feed them to that viable level they will get aggressive imo.

  2. Post
    Al Ulrich


    Thanks for reading and posting. I think you have some valid points there about aggression. I guess my point on feeding was based on observations that aggressive fish tended to fight over food–essentially trying to exclude the subordinate fish from eating–and I observed (non-scientifically) that a fat and satiated fish (that used to be aggressive) takes a break when digesting his or her meal.

    As far as breeding–I completely agree. Territoriality and breeding seem to go hand-in-hand (or fin-in-fin)

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