Fish to Avoid Adding in Your Aquarium


Six-Line Wrasse:

This is one of the more commonly seen wrasse in local fish stores. They are cheap, and their colors usually draw people in to buying them. However, that comes at a price. These fish can become aggressive at the flip of switch to other fish around its size. They will also eat any and all plankton or micro-foods given to corals for sustenance. This can be devastating overall to people who own smaller aquariums. With the six-line wrasse establishing territory issues, and consuming all the food it can find, this fish is best kept out of one’s aquarium.




This is the cheapest fish found on the market, and for good reason. Damsels are incredibly aggressive and will fight any fish around their size. Some damsels, like the domino damsel, will even pick fights with larger fish and will force other tank members into hiding. They are completely resilient, which comes through their natural instinct. In the wild, damsels will farm and consume seaweed. Their natural instinct is to keep all fish away from their territory if possible, and they carry that trait with them even while captive bred. With that being said, I would advise avoiding fish marked as “damsels” at local fish stores. If you are needing a hardy fish to help cycle a tank, then two other fish in the same family are better suited for this task; ocellaris clownfish and chromis.




I listed this fish in one of my previous articles about toxins. This fish not only gets extremely large in full adulthood, but they are often purchased for being “cute” or unique. If this fish gets stressed, then it will emit something called ostracitoxin, which can wipe out your entire aquarium within minutes. This toxin cannot simply be washed out, either. It will stick to the adhesive in tanks and go into your filtration. This fish is not worth the risk at all in my opinion.




Lionfish have to be one of the most stunning and diverse looking saltwater fish you can find. However, there’s a couple reasons that they should not be kept in aquariums unless you know what you are getting into. Lionfish tend to be reluctant at eating in captivity, and will sometimes even shy away from prepared frozen foods. They have also been known to eat smaller fish in aquariums if they feel that they are not getting sufficient nutrition. Oh and did I mention that their spines are highly toxic? This was also discussed in my toxins article, and a lionfish can produce enough neurotoxin to send you to the hospital if you get pricked. I have always wanted a lionfish in one of my aquariums, but I know well enough that the reward is not worth the overall risk.


Mandarin Goby / Dragonets:

These fish are remarkable to look at. Their colors are so enticing that many people are drawn into getting one due to not only its patterns, but also its smaller size. This fish comes with a few issues, though. The mandarin goby has got to be on my top 5 list of fish that picky eaters in captivity. Most would rather starve themselves to death than eat what you are preparing for them, if feeding them incorrect foods. The foods they tend to eat are copepods or any type of smaller shrimp. I owned one of these guys, and the only way that I could get him to eat was when I used a syringe and injected mysis shrimp into the sand bed itself, along with filling the aquarium with pods. They typically scavenge the sand for food, and use their gills to ventilate sand out of their bodies. Incredibly cool fish, but I would only recommend this to an elite aquarium owner, one who has the time and patience to start a copepod colonization in their tank. A standard aquarium size for one mandarin goby alone should be about 100 gallons.


Cleaner Wrasse:

This fish is luckily harder to find in fish stores these days, which is a good thing. A study was done a while back that shows approximately 90% of reef fish have left their homes and cleaning stations once the cleaner wrasses were removed from coral reef beds, due to being sold to aquatic stores. In the wild they are not necessarily picky eaters, but they have a unique requirement when eating. Their primary food source is parasites and bacteria build up. In cleaning stations on the Hawaiian seabed, they will attach to fish and collect their food this way. An aquarium simply does not, and cannot produce enough of both of these things for the fish to survive. Out of desperation, they will begin ripping holes in your fish’s scales to find food. They are also notorious fpr jumping out of aquariums. If you are wanting a fish that will help clean off your other fish, then a sharknose goby or neon goby are better options. This link written by our owner goes into more depth about them.


Moorish Idol:

Before anyone says anything, yes I owned one of these fish and yes it was the coolest fish I have ever owned. With that being said, I knew exactly what I was getting into before I got it and set up my tank to revolve around this fish. The reason that they should be avoided for most people is that they are another fish that are incredibly reluctant about eating, and require too many oddities in their diet. Most flat out refuse to eat regardless of what you have, simply because they would rather die than be kept in a box of water. If you do manage to get one that wishes to survive, then a multitude of food preparations are needed for its sustainability. They require frozen and flaked foods, almost at rotational feedings. They also graze on seaweed throughout the entire day, so buying packs of seaweed at a time are expected. One last thing they require in their diets are sponges. You have to introduce sponges into your tank and let them manifest on the rocks, which can take a long time. This fish is also susceptible to ich outbreaks, so you have to be very careful about what you are doing in the tank.

Harlequin Shrimp:

These little guys may look beautiful, but that comes at a price. While they are not commonly listed on other sites to avoid, I am adding it here with reason. Harlequin shrimp have one food, and one food only that they consume; echinoderms. That is the scientific name for star fish and urchins. If you are not prepared to constantly go to the store to purchase star fish, then don’t think twice about this crustacean. Star fish are not inexpensive, with the cheapest being a chocolate chip sea star ranging around $10 a piece. Also you have to determine if you are mentally prepared to have a shrimp continuously eat a living creature at a slow pace… as the echinoderm sits mortally wounded until finally being completely consumed by these little guys. I have known people who bought these shrimp and later regretted it. If you are planning to own one, though, then please try to share it with fellow aquarium owners in their tanks. This is so that they can eat small portions at a time from each aquarium and not completely kill off the echinoderms. That way the sea star, or whatever you are feeding the shrimp, will have a chance to regenerate their missing arms and not die off.


Aside from the fish listed above, please do your research about any other fish that you are wanting to add. I know some can look enticing, but the ones that usually have ornate colors will typically have a catch 22 that goes along with them. If you are unsure about a fish, internet searches are your friend, along with knowledgeable fish salesmen. Please don’t do something like get a triggerfish and add it to your reef aquarium. Or a pufferfish added to a tank full of prized invertebrates. Despite how alluring some fish may seem, think about what is best for the fish first before any rash purchases.