Eric is an aquarium enthusiast with over two decades of experience caring for a wide array of tropical fish.
Fishkeeping is a rewarding hobby once you have the hang of it. Unfortunately, every new aquarium owner has some hard lessons ahead of them before they get things right.
You can’t beat yourself up when you do your best and a fish dies anyway. All you can do is try to figure out where you went wrong and learn from your mistakes.
But there is one habit you can get into that will increase your chances of success:
Learn everything you can about a fish before you bring it home.
Our smartphones give us access to information about any fish, right at our fingertips. Yet, people still make impulse buys in pet stores and end up with fish they can’t manage.
It’s an avoidable error. Take your time, do the research, and choose fish that are appropriate for your tank and compatible with the fish you already own.
You may be surprised to find out that several popular fish aren’t a suitable fit for most home aquariums. Pet stores continue to stock them because inexperienced aquarium owners buy them on a whim without knowing what they are getting.
In this article, you’ll learn about seven fish you probably ought to avoid.
1. Common Plecostomus
This is the “suckerfish” you see clinging to the aquarium glass and hoovering up algae. Well-meaning aquarium owners purchase these catfish in an effort to keep their tanks clean.
Plecos are indeed prolific algae eaters, but they come with a handful of problems most people don’t know about until it is too late.
The main issue is this: The common pleco grows to an adult size of nearly two feet and is not appropriate for most home aquariums. In fact, I’d go with a tank of at least 150 gallons for one adult.
Plecos require driftwood to rasp on and can become more aggressive as they get older. They will lash out at other fish at feeding time if they aren’t getting enough to eat, so it is smart to supplement their diet with algae wafers.
There are better algae eaters for most aquariums. Smaller species such as the rubbernose and bristlenose pleco only grow to four or five inches in length.
Read More From Pethelpful
Or, you could opt for a school of otocinclus catfish. Otos are only about an inch long, eat algae, and stick to the glass like tiny little plecos. These guys are something called "dwarf suckerfish".
2. Green Spotted Puffer
I’ve seen green spotted puffers listed as freshwater fish in pet stores, along with a warning that they “may nip fins”.
But here is the cold, hard truth about the green spotted puffer: They are little killing machines.
Green spotted puffers are aggressive, territorial predators that rarely tolerate other fish in their tank, even other puffers. They can live in freshwater as juveniles but must acclimate to brackish water as adults. Some puffer keepers even house their fish in marine setups.
GSPs won’t eat fish food. They need meaty foods like bloodworms and krill. They have boney plates in their mouths that grow continuously. Puffers require a constant supply of live pond snails or similar hard foods to keep their “teeth” trimmed.
I’ve kept green spotted puffers successfully and they are among my favorite fish, but you’d better know what you are getting yourself into if you bring one home.
3. Iridescent Shark Catfish
This is a beautiful fish you will see in pet stores as a four-inch juvenile. Because they are so cool, and because they are called sharks (even though they are a type of catfish), they’re quite popular. But, as with the other fish mentioned in this article, there are a few things most people don’t know about them.
Iridescent shark catfish are schooling fish, so you’ll need a group of six or more. Otherwise, they become skittish and stressed.
They also grow to an adult length of nearly four feet.
So, what size tank do you need for six fish, each four feet long? A thousand gallons, maybe? Who knows?
Unless you have an enormous aquarium or a secure outdoor pond in a climate where this fish can thrive, it is best to give the iridescent shark catfish a pass.
Note: When I say a secure outdoor pond, I mean a pond where there is no plausible way for your fish or their offspring to escape into the wild. Part of being a responsible aquarist means ensuring non-native fish and plants don’t get a foothold in environments where they don’t belong.
4. Bala Shark
Bala sharks are not really sharks either, of course. They are in the family Cyprinidae which includes carp, barbs, and true minnows.
They are beautiful fish, with their shiny silver scales and sharp black-and-white accents, and they are hard to pass up as cute little three-inch babies.
But the bala shark is a powerful, fast-moving, schooling fish best kept in groups of six or more. They spook easily and there are even tales of startled adult bala sharks cracking aquarium glass.
And they grow to an adult length of over a foot.
If you intend to keep bala sharks, you’d better be prepared to house six active, foot-long fish. I wouldn’t put them in anything smaller than a 300-gallon tank.
5. Tinfoil Barb
Here is another pretty fish that’s a relative of the bala shark. Tinfoil barbs also grow to over a foot as adults, a fact that no doubt has caught many novice fishkeepers by surprise.
Like the bala shark, the tinfoil barb is a large fish that thrives in schools and requires at least a 300-gallon tank.
Much of what applies to the bala shark also applies to the tinfoil barb. Neither is appropriate for most home aquariums.
What could be wrong with goldfish? Kids win them at carnivals, for goodness’ sake.
I bet there are a few things you don’t know about goldfish. For starters, those tiny fish kids win at carnivals then bring home and keep in a bowl are common and comet goldfish. Both can grow to a foot or more as adults.
Fancy goldfish — the ones with wild flowing fins — can reach eight inches when they are mature.
We’re going to need a bigger bowl.
Speaking of bowls, because of their physiologies, goldfish pollute their water in a hurry. That means a fishbowl fast becomes a toxic soup for your little hard-won carnival fish.
There is nothing wrong with keeping fancy goldfish, as long as you have a large enough tank and understand goldfish care. However, in my opinion, comet and common goldfish belong in ponds as adults.
7. Silver Dollar
These round, shiny fish are relatives of the notorious piranha, believe it or not. But don’t worry; the silver dollar won’t reduce the rest of your fish to tiny little skeletons.
Silver dollars are peaceful omnivores that pose much more of a threat to plants than they do fish. They’re schooling fish and very active swimmers who spend their time in the upper part of the water column.
The problem is that silver dollars grow to about six inches in diameter. You’ll need a large tank to house a school of six — at least 150 gallons. Because they are so active, you’ll want a long tank rather than a tall one.
It’s a better situation than what we’d face with the bala shark or tinfoil barb, but still more than most aquarium owners are prepared to deal with.
Ethical Fish Keeping
If you are an expert at aquarium care there is no reason you can’t keep any of the fish listed above. I’d guess you already know about the issues surrounding them and you’re ready and able to deal with them. You probably don’t need any advice from me.
But most fish keepers aren’t experts. Most are somewhere in the beginner-to-intermediate range. In my opinion, those folks would be smart to stay away from these seven fish.
If you have already made the mistake of stocking a fish that is getting too large for its tank, take the time to find a home for it. Sometimes pet stores will take them, or even local enthusiasts. With a little effort, you should be able to rehome your fish.
I believe aquarium owners should strive to be the best stewards they can be for the animals under their care. Ultimately, it is up to you to make the right choices and care for your fish correctly.
What about the pet stores themselves? How much responsibility do they have to make sure customers are informed? I would think a great deal, but it doesn’t always work out that way.
Some employees are very knowledgeable where others don’t have a clue. In some big-box stores, the same employee who tends to the fish might also handle other departments like sporting goods and electronics.
We can’t control any of that, so don’t rely on them to help you make decisions. Do your own research before you even set foot in the store.
It would be even better to find a small-scale local pet store run by people who are knowledgeable about fish. These shops are rare these days, but I think it is worth it to give them your business if you can.
This article is accurate and true to the best of the author’s knowledge. It is not meant to substitute for diagnosis, prognosis, treatment, prescription, or formal and individualized advice from a veterinary medical professional. Animals exhibiting signs and symptoms of distress should be seen by a veterinarian immediately.
Eric Dockett (author) from USA on September 29, 2020:
@Suhail - None of these fish are harmful if cared for properly. Aquarium owners simply need to understand the requirements of the fish they purchase.
Suhail Zubaid aka Clark Kent from Mississauga, ON on September 28, 2020:
Very helpful article! I did not even know that some common home aquarium fish are actually harmful.