How to Remove Unwanted Fish from a Reef Tank

Okay let’s admit it, we all make mistakes. And sadly I must admit that I probably make more than most. As we all know there are two types of mistakes: omission where we don’t do something we should have, and commission where we actively do something that causes the problem. Adding a fish to our tanks is unique in that it can be a mistake of both types.

It may be omission, in that we did not do our homework before adding it or we didn’t realize that it grew so big or would eventually eat all of our prized corals, etc. And the mistake is always one of commission because fish do not suddenly just appear in our tanks. Regardless of which type of mistake it is, most of us at one time or another have added a fish that we regretted adding to our tanks.

It could be a cute blenny that the LFS swore would not mow down our corals, or an angel that we had for years, that suddenly went rogue and decided to eat all of our LPS or a fish that we knew was risky, but we knew that if we kept it well fed there would be no problem. The reality of this hobby is that this happens to all of us, and usually more often than we like to admit, so once we acknowledge that there is a problem, what do we do about it. For even in a small nano tank, it is darn near impossible to get out even one fish without tearing down the entire tank and for a big reef tank you may be looking at entire day if you need to tear down the tank.

As a result of countless mistakes I have made in adding the wrong fish over the years, I have mastered the art of removing unwanted fish from even the largest most coral filled tanks. Even though I love saltwater fish almost as much as corals, I do stick to the rule that if a fish starts eating my corals out it goes. Through doing this I have come to realize that fish are far smarter than me when it comes to not being caught but they are not as patient.

With some ingenuity and a lot of patience I have been able to remove almost every troublemaking fish that I have added to my tanks, without having to resort to tearing down the entire tank. Doing this does require that you understand the fish’s behavior in order to figure out the best way to catch it as there is no one method that works for all of them. But it can be done.

We will start with the easiest method and the one I use most frequently: the fish trap. Everyone who has been in the hobby for any length of time has seen this big box with a sliding door that in the best case the fish will swim into where they will be trapped. Oh if it were only that easy. Sure some fish are not that bright, but are curious, so they will swim in right away to be trapped, but many will not.I could not figure out why and realized that if you look at it a fish trap looks like a big open mouth. And like I said fish have proven smarter than me, so they do not swim into a big open mouth. So to try and remedy this problem I do several things.

First I get the biggest trap I can that will fit into my tank without knocking down all of the corals. I then glue pieces of coral, rocks and gravel around the opening so it does not look like a mouth and third I plan on this taking at least a few days to work. On the first day it just rests or hangs in the tank to allow the fish to get used to it. On the second day all food that is put in the tank is put in the box. For this either a feeding chimney or a baster is used so that the food is at the back of the trap.

On the third day the door is placed on the trap and adjusted so that lowering it can be accomplished quickly. Also during this time you need to get the fish accustomed to your being near the tank and near the trap so stay near the trap as you feed and keep your hands near the door, otherwise the fish will bolt if they are not used to your presence.

Once the fish are accustomed to all of this food is introduced on a regular basis and in the best case the fish I want to trap will go in to eat and be trapped and removed. Sadly this is the exception rather than the rule with using this type of trap, because for some reason fish just inherently know that this is a trap. That may sound funny, but I have no other explanation for why this doesn’t work more often.

So since that did not work as often as I would have liked, I developed a variation of the trap that I call the basket. For this method a mesh basket, like that used for ponds that can be picked up at the big box hardware stores, see picture, is placed in the tank. Two magnets are used, one inside the basket and one on the outside glass hold the basket in place. The basket is placed about a foot or so below the surface of the water and as with the trap all food for the tank is placed in the basket.

As the fish get used to feeding from the basket and my presence the basket is gradually moved up higher and higher in the tank so that eventually it is only 4-6 inches below the surface. Again not all fish will go in it, but if the fish I want to remove does then all that is required is for the basket to be quickly lifted to the surface once the desired fish is in it. I should point out that as with the trap you usually only get one shot at this for once the fish know this is a trap they will not enter it a second time. So that is why patience is key here as the fish will dart out much more quickly if it does not feel comfortable in these traps.
Unlike the above methods which simply rely on bait and a trap these next methods require more effort. The first is a technique I’ve developed for use in my nano tank and for when I know where a small fish, like a coral eating goby or a pugnacious pseudochromis, sleeps or likes to hide. For this method a long piece of 1” diameter flexible tubing is needed as well as a red LED flashlight.

The first step in this process is to see where these small fish sleep or where they like to hide. To determine this, I visit the tank after all the lights are out and use the red flashlight to see where the fish are without their being able to see me. Generally these small fish are very territorial so I check out the tank for a couple of nights at least to make sure that they are in the same spot each night.

Once I have determined their favorite spot, I get the hose ready by starting a siphon in it and either having it flow into the sump or into a large bucket. Due to its large diameter the hose will siphon water from the tank very quickly so make sure to use a bucket of adequate size. Once the siphon is started I stand on the hose and again locate the fish with the red flashlight. The hose is then directed to where the fish is resting, my foot is lifted from the hose and if all goes well the fish is quickly siphoned from its lair.

There are several keys to this going well. First make sure that the fish is in a good position to be drawn in along its long axis rather than pulled sideways into the hose where it will be injured or killed. Second try not to bump anything as you move the hose as often even a slight vibration will cause the fish to move out of position. And third stop the hose while the fish is trapped and put it into a suitable container because if you let it flow into the sump you are back to trying to catch a fish that is more adept at evading capture than you are at catching it.
I also use another technique for when larger fish sleep above or in front of the rocks and corals in my tank. Again the first step is to assess where the fish sleeps and to make sure you have clean access to it. Once this is ascertained, again using the red LED flashlight, get a net ready just slightly larger than the fish and get it as close to the fish as you can.

Once it is close switch on the brightest white light that the flashlight provides. This bright white light will temporarily blind the fish and kind of stun it like a deer in the headlights so it won’t move. This will provide enough time to net the fish quickly and remove it from the tank. I have used this technique to remove aiptasia eating filefish as well as some aggressive wrasses and tangs.
If the above listed techniques don’t work I then resort to more “nuclear” options. While not as nuclear as tearing down the tank they are a bit more traumatic to the tank and the fish. In the first method clear plastic is fastened to the first piece of pvc pipe that is at least as tall as the tank. The clear plastic is about two-thirds as long as the tank and is fastened to a second piece of pvc. I use gel super glue to fasten the plastic while everything is dry.

The two pipes are placed so that one piece is firmly against the far side wall of the tank and the second is placed so that it will act as a partition between the front glass and the rest of the tank, with the opening being like a large V. I usually find this works best if the flow within the tank is reduced or shut down while this is being done.

To prevent the fish from escaping through the bottom plastic, small pieces of live rock or coral are placed along it to keep it from moving up. As with the other types of trapping, all feeding is done in front of the plastic so that the fish have to be within this partitioned area in order to eat. Once the desired fish swims into this area the moveable pipe is pulled to the front glass closing off escape and the fish can either be held against the glass in the plastic or it can be netted out from the partitioned area.

Like I said this is a more traumatic means for catching an undesired fish. Sometimes even this does not work as advertised so a bit of ingenuity may come into play. For instance, one angel that I tried everything to catch simply would not be fooled with any of these methods. I was removing him because he suddenly developed an appetite for large fleshy lps. So since I had run out of ideas I tried placing a couple of open brain corals in the front in front of the plastic. Amazingly within 10 minutes he got over all of his shyness and swam into this partition trap so I could finally remove him.

So if all of this fails and you still do not want to tear down the tank there is one last option that I have only used once, but have heard of its use many times. The old bait on a hook method. In order to do this you need to get the smallest hook you can find. Remove the barb from the hook, as you don’t want the hook to set and damage the fish’s mouth and then find bait that the fish finds appealing.

In this instance you then need to attach the hook to monofilament line and then get the hook to be near your intended target. However, unlike normal fishing this is kind of the opposite. Since most of the fish in your tank are not scared of hooks and are always hungry they will attack the hook if you aren’t careful. So in this form of “fishing” you actually have to try and keep the hook away from the other fish and get it near you target. Needless to say this requires a lot of focus and patience and it can be quite tiring. As I said I only used it once to remove some incredibly aggressive and smart damselfish and it took almost half a day to do so.
Hopefully none of you will make the same or at least not as many mistakes as I have made. However if you do by adding the wrong fish to your tank at least you now know that there are ways to remove them without tearing down the entire tank.