Bringing home a new fish, whether it's as common as a green chromis or as rare as a peppermint angelfish, it is always exciting with a dollop of stress. Stress for the owner hoping they eat what you feed, get along with others, or that it won't disappear in the tank leaving you to wonder if it is even still alive. It can also be stressful for the fish too as it gets used to its new surroundings and even more so if it is being chased or nipped at by others. So, if you are worried that the new fish might be dealing with bullying, here are a few methods hobbyists have used that can help when dealing with aggressive tank mates.

Acclimation Boxes a.k.a. The Penalty Box

acclimation box for new fish allows aggressive fish to get used to its presence

Installing a temporary acclimation box into the aquarium so everyone gets acquainted, is good practice. Setting the new fish in one of these helps introduce your new fish or even coral to your tank, safely. It will allow you to observe how all the other inhabitants react, noting which ones may end up being problematic. So, plan ahead in case of potential troublemakers (if needed).

If you do see problems arising, some acclimation boxes second as a trap. Hostile fish can be lured in with some food and housed in the penalty box to serve their sentence until you figure out what to do with them. The aggressive fish can be banished to a refugium for a bit, otherwise, your LFS may help you out by taking them back and may even offer store credit or a discount. Otherwise the box can house the new fish again, giving them time to heal up if they were being picked on.

The Mirror Trick

placing a mirror on the side of the tank can trigger the aggressive tank mates to attack its reflection instead of the new addition

For aggressive tank mates, they might not like the new fish addition very much, but the truth is they probably hate themselves even more. In other words, the attacking fish would usually act more aggressively towards their own kind than they do a new fish from a different species, seeing them as competition of: food, breeding rights, and territory. To take advantage of this try placing an image of the aggressive fish against the tank. The "mean" fish should try to start asserting dominance toward a picture of its own species instead of the new fish. You may even take it a step further and place a mirror upon the glass for a short period, until the new fish settles. This method has been proven effective with known aggressors like tangs, clownfish and some wrasses.

Rework the Aquascape

adding new rockwork or rearranging the scape can disorient any established fish

Many established fish have probably claimed a territory in the tank which they will protect from any invaders. Sometimes rearranging part of the aquascape within the tank will disorient the aggressive fish enough to take their attention off your new fish. Aggressors theoretically focus on reestablishing themselves in the rearranged part of the tank. Adding new rock can help as well, giving more places for fish to hide when they feel threatened.

Another Aquarium...?

moving to a larger aquarium can decrease aggression allowing more space for aggressive tank mates to claim and swim away
Reefer Deluxe XL 200 Aquarium - Red Sea and NUVO INT-200 Internal Overflow Aquarium - Innovative Marine

If you feel like you want to keep both fish and hate to see either one in the timeout box or even banished to the sump, it might be time for a bigger tank. If you were planning on one, the more room for your fish to swim typically reduces tensions and allows others to escape into a new zone, if the aggression continues. The new aqua scene will also act as a reset button for every fish to establish a new territory, if you are dealing with aggressive tank mates.

For more info on moving, check out our other post: Ready to Move Into a Bigger Tank? Tips to Prep for the New Tank

Feeding Technique

Easy Masstick fish food sticks to different surfaces when feeding

Like the mirror trick, feeding the tank's favorite food can distract any would-be aggressors as you add the new fish. Feed some of the fishes' favorite foods, in smaller portions in multiple intervals when you check up on the new addition. Be mindful of not overfeeding and of the drawback of not being able to feed the tank all the time. So keep an eye on the established fish after they are done eating to see if they noticed the new one.

Multiple Fish Additions

Adding more than one new fish to the aquarium is also a technique of confusion. If fish you wish to add does better in groups or if there are more than one fish you would like to add, it can lessen aggression from established members of the aquarium. Of course, this is based on if you have enough room for the fish, and if your bio-filtration is mature enough to handle the new load.

Don't forget a Tank Lid

aggressive tank mates may chase new fish, causing them to jump, so a mesh lid may be needed or considered
AUQA Gadget DIY Mesh Screen Lid Kit - Innovative Marine

In many cases aggression typically lasts a short period of time ranging from a day to a week as the fish establish their pecking order. During this time, the chasing can trigger a fight or flight situation for the new guy. If you think that it can end up in a flight situation - a one-way ticket to a bungee jump... without the bungee, to your floor - consider placing a lid on the tank, temporarily.

Final Thoughts

These are a collective of techniques which have worked for many hobbyists, freshwater and saltwater, when dealing with aggressive tank mates. Keep in mind that not all fish will take to these techniques and that many fish are just aggressive by nature, so plan ahead for any new fish additions. Research the fish you plan on getting, if they are known to be solitary or schooling, and how much tank room they would need.

If you have tried any other methods when dealing with aggressive fish, feel free to let us know - spread the knowledge - cheers!

Check out our Fish Compatibility chart for future fish consideration.