IMPULSE FISH--Gentle Giants


Jans Balas

One of the most important factors in becoming a successful hobbyist is to find out what kind of fish you're looking at BEFORE you buy it. If you don't do the research first and buy a fish you know nothing about just because it's cute, you may be in for many unpleasant surprises down the road--not the least of which is how big that cute little fish is going to get.

Jan, the lovely lady who was so kind as to share this article with us, knows whereof she speaks; she owns several of these "gentle giants" herself. Her guys lucked out; Jan has done what it takes to house them properly and has expanded their living space over the years as they've needed it. Moses, her common pleco, who's now nearly a foot and a half long, has been with her for over twelve years--which emphasizes yet another thing to consider when you buy these big fish. If you take good care of them, they'll live a long time and reach their full size, so be sure you're prepared to make a long-term commitment!


Many of us have made the mistake of buying an "impulse fish" at least once in our lives--you know, the ones that you see in your fish store, that are so tiny and cute. You ask the store clerk how big they get, and they tell you that they can get big, but they are slow growers. Some will even go as far as saying that they will only get as big as your tank allows, or they will flat out tell you that they don't get very big (because they don't know). Wrong! The only reason that they usually don't reach their full size is that they die long before they get to that point as a result of being kept in poor or inappropriate conditions. The bigger they get, the bigger the load on your filtration system, and the more frequent and larger your water changes have to become in order to maintain them.

One of the first steps in being a responsible fish keeper is to sit down at home, read every book you can find, go on the internet and do extensive searches, or ask someone who has some experience with that particular type of fish, before you buy it. That step alone will save you (and your fish) more problems than you can imagine. Sure, some fish do grow much slower than others, but you have to look at the long-term commitment. When you buy a fish that is 1" long, you can't figure that you can put 10 of these 1" fish in a 10-gallon tank. (That 1" per gallon rule is not a very good gauge of the fish capacity for your tank, by the way.) You have to look at the adult size of these fish, and the body mass of each one. Discus or Silver Dollars, for example, have bodies as deep as they are long. A few years down the road, those 10 1" fish may easily turn into 150" (or over 12' of fish!).

So many people say "Oh, it will take a long time before this fish gets too big for my tank, I'll deal with it then." "Then" comes much faster than you can imagine. Just as one example--an oscar that is properly cared for can and will easily reach 8 or 9 inches in body size in its first year. At that point you will either need to find a spot in your home to put a bigger tank, and have the funds to do so, or try to find a new home for this fish. Any fish keeper who does have a tank big enough to handle some of these giants is going to already have it devoted to the type of fish and aquascaping that they intended to buy such a large tank for. At least 90% of the time, there isn't going to be anyone who will take the fish who has outgrown your tank, including public aquariums and zoos (they get more requests in a year to adopt these big fish than they have gallons of water). Then what? Taking them to a lake and releasing them (which happens far too often) isn't an option --or at any rate, it shouldn't be, either from a legal or an ethical standpoint. If you opt for this solution and get caught, you better not have any plans for the next several years, because depending on where you are you may wind up in jail, and/or have to pay a hefty fine--not to mention the fact that such an irresponsible act completely disrupts the natural balance for the native fish and plants in that lake, river, pond, etc.

In the mean time, this nice fish that is growing, and starting to become very uncomfortable in the small tank that you have put him into as a baby, doesn't have much of a future. They have done nothing wrong, and still face the fact that they are probably going to die far before their time, and lead a very poor quality of life until that time. Is this really your idea of being a responsible fish keeper??

Here is a very small list of some of the most common fish sold in stores that will get far too big for your average tank:


Common Plecostomus

Adult size up to 24", and life span of 25+ years. at nearly 18" and more than 12 years old, Moses, pictured at right, is well on his way.

Common Plecostomus

Oscar (Astronotus ocellatus)

Adult size 14" or more, and is almost as tall as he is long. And if you're thinking it will be years before he gets that big, forget it--an oscar can easily reach 8 or 9 inches in body size in its first year. The minimum size tank you'll be looking at is a 75 gallon; it won't be long before he won't even be able to turn around in a 55 without touching the front and the back at the same time.

Red Tail Catfish (Phractocephalus hemioliopterus) - Adult size is four to five FEET. Nothing less than a tropical pond will do for these fish. These are not the Labeo sp, which are sometimes called a "Redtail". The fish in this picture is well over four feet long, and a tankmate of the pacu at the Tennessee Aquarium. For some perspective--his whiskers are about 18" long. Red Tail Catfish
Pacu Pacu - There are several species, but one such as the brown pacu (Myleus) grows to 24" long and tall. Even if you have a larger tank that is 24" tall, you are going to have dorsal fin sticking out of the water! The guy above is a resident at the Tennessee Aquarium in Chattanooga who's about two feet long, but still growing--as he gets older, his body will become even deeper.
Bala Shark

(Balantiocheilos melanopterus)

Adult size 14", and needs lots of swimming space, as they like to swim long stretches at a high speed... This fish needs to school, so you will need to plan on having at least four 14" fish in your tank. This fish is not a shark at all, but a member of the Cyprinidae or minnow family. They are beautiful fish, and very appealing when they're sold in fish stores as babies of just a couple of inches long--but do you see those guppies at the top of the tank? They are NOT going to stay that size.

Bala Shark
Silver Dollar Silver Dollar

(Mylossoma species)

There are several different species of Silver Dollars, but many of them reach at least a length of 8", and again are almost as tall as they are long. They also need to school, and are very skittish, so in anything less than a large tank, they will continually bump their noses on the glass, and just not act naturally unless they have the room to form a natural school. Don't ever plan on having a live plant of any kind in your tank with these fish. They will devour even the toughest plant.

Synodontis Species

While there are several of them that do stay small and work well in smaller tanks, there are many that get huge and aggressive. A couple of these monster sized ones are the Syno Longirostris which can reach an adult length of 24", and the Syno Acanthomias which can reach a length of 16" or more. Again, do your research before you go shopping. Don't walk into a store just looking for a Synodontis, and take the first one labeled that on the tank. You may get a three inch fish--or you may wind up with a fish that's going to be two feet long when it grows up.

The gorgeous fish at right is Synodontis eupterus (commonly known as the featherfin), one synodontis species that can be kept successfully in the home aquarium. Take note, though, that even this relatively small species requires a good-sized tank; the guy at right has a body length of seven inches (thanks, Dawn, for letting us borrow Moby Dick!). Since they prefer to be kept in groups and need swimming room, unless you have at least a 55 gallon tank, they'll wind up cramped, too.



There are many different types of goldfish, and they reach many different sizes (some of which are huge). They deserve mention here because they have falsely earned the reputation of being "bowl fish". These are big fish with a huge waste load, and need much more maintenance than most fish; unless you're prepared to give them the 20 to 30 gallons apiece they will need as they grow, you might want to think twice.


Most of the fish listed above will prevent you from having any kind of planted or specialized theme; because they are so big, they will rearrange your aquascape with just a flick of the tail. The kind of tank you wind up with will be up to them, not you.

This list is just a fraction of the fish that shouldn't be kept in anything but large aquariums (75 gallon +). It does not cover the many other issues involving buying impulse fish, such as compatibility; that's a subject for another article.

I wanted to try to get some of these points across because I have made many of these very same mistakes along the way, and if there is just one person who will take it to heart, that's a step in the right direction. Luckily, I have had the space and funds to get bigger tanks to accommodate these fish, but in the long run, if I were again setting up a tank such as my 125 gallon pictured below, there would be many other stocking or aquascaping choices that I would rather have made for it. While I love those gentle giants, and they will live out their life with me, it is a lot of work to keep up with their huge demands--something that won't be an issue for you if you do your research and think about the long term needs of any fish before you buy them.

Jan's 125

Jan's 125--home of Moses, the four balas and the silver dollars.

There are a million different kinds of beautiful smaller schooling fish that can make a stunning display in your home. Give them a chance!

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