The 15 Worst fishes, that are not for the Beginners

All beginners in aquarium husbandry often grope in the dark not knowing what fish exactly they want to have in their tank.On seeing a small and cute suckermouth catfish in a pet-shop they don’t even have a clue that it’ll grow to be over 30 cm (12 inc) long and live over 20 years.

And this graceful oscar fish (Astronotus ocellatus) will grow to be very large and it’ll eat everything in a tank that gets into its mouth.

So, what fishes is it better to avoid at the beginning?

So, below I’m going to list 15 fishes, which are popular and at that very demanding in care (also here you may find the list of 10 best fishes for the beginners and top 10 unusual tank fishes).

If you are the beginner aquarist, so it’d be good to avoid these fishes, at least till you get some experience in the field of aquarium husbandry. Then you’ll surely be able to create necessary conditions in your community tank or even get a separate tank for these demanding fishes.

With no doubt each of the fishes mentioned below isn’t the most simple in care and it needs much more efforts to provide it with a proper care, than any other common tank fish.

As a rule these fishes require some special tank water parameters or strong filtration, or they may be aggressive, destroying everything in a tank, or more often these fishes grow to become huge and they need very spacious tanks, which is also a problem sometimes.

Black pacu

This is a herbivore relative of a well known piranha. These fishes are from the same Characidae family. The size of reproductive black pacu species makes it different from piranha. And here come troubles for the beginners.

The majority of pacu species you may see on sale are not larger than 5-7 (2-3 inc) cm long, they have nice coloring and peaceful temper. However, 200 liter capacity tank will be small for the fish during the 1st year of its life and it’ll continue growing to become about 40 cm long and weigh about 4 kilos.

Sadly, their bright coloring later gets pale.

So, I’d say – avoid this fish by all means, unless you have a tank of 1-2 tons capacity hidden somewhere in the house. The same applies to all other fishes, if asking about their size you get some mumbling instead of the answer.

The red-tailed black shark

The redtail shark is very popular among aquarists due to its nice and bright coloring, the shape of its body, that reminds a shark one. The fish appeared in our list not so much due the difficulties in care as due to its high territoriality.

The thing is that this redtail shark can’t stand any other tank mates which have alike coloring and it gets even worse when there are some related species around. If you choose redtail shark, you have to keep it with other large fishes with different coloring, since otherwise red-tailed black shark will haunt and beat them.

Keep in mind, that it grows to be rather large and its attacks may be rather harmful for other fishes.

Leopard pleco

If you have troubles with algae in a tank, then you should get this fish. It’s much easier to get leopard pleco, than to study what is wrong in your tank.

You can see this fish on sale quite often and it’s not a problem to buy it. But again the fish sold in a pet-shop will be about 7-10 cm long and it’ll grow. It’ll grow a lot.

The majority of beginners start with a tank of about 100 liters capacity. To put a Pterygoplichthys into such tank is like to put a killer whale into a swimming pool.

The fish grows to be 30 cm long and more. It’s easy to guess how much space they require and how much waste products they leave.


Another fish that is often on sale as well as Oscar fish, which has good-looking black and orange coloring and attracts a lot of attention.

This fish requires a tank from 200 liters capacity to 300-500 liters in the wake of rising. This is definitely not the fish anyone wants to be the first in his tank.

Besides, oscar fish will eat any fish that fits into its mouth, sometimes it’s even fed with goldfish and other small species. For the beginners oscar fish is too large and too aggressive. It requires a separate big tank for only large species.

African cichlids

It is one of the most beautiful fishes in a freshwater tank. The problem is that the fish is highly aggressive.

Quite often the beginners don’t know about it and they may have lots of trouble in their community tank because of the african cichlids.They may kill the majority of their tank mates and fight each other as well.

Although african cichlid is a very good-looking fish, it’s better to leave it for some experienced aquarist due to the fish territoriality and the fact that it needs special care and frequent water renew.

Silver arowana

Another one fish that is often sold to the beginners without warning them about its adult species size.

Silver arowana as well as oscar fish will eat everything that it can swallow, it requires big and long tank (at least 3 fish length long and the fish grows to be about 1 m long). Graceful and interesting fish in its teens, arowana grows into a large monster with the corresponding appetite.

Bala shark

Another fish that looks like a shark, but actually it’s a carp relative. Bala shark grows to be about 25 cm long and it’s a schooling fish as well, so you’ll have to keep at least 5 species in a tank. The fish is definitely not for small tanks first of all because of its size, although it’s rather peaceful.


As well as african cichlids discus is one of the most beautiful freshwater fishes. It’s very peaceful and calm, but requires special tank conditions and closer attention.

Higher water temperature, frequent water renew, cleanness, special diet and necessity of large swimming space make this fish rather complicated for keeping.

The fish feels best in a separate tank where all above mentioned conditions are fulfilled. Again, it’s better to buy discus fish when you are already an experienced aquarist.

Glass catfish

The glass catfish looks very awesome having a transparent body of unusual shape. But the fish is a nocturnal one, it’s very sensitive to bacterial infections and it should be kept in a school of at least 6 species.

In case of any tiny change of tank conditions the fish tends to die. So, these are the reasons why not to get this fish when you are a newbie in aquarium husbandry.

Oto catfish

It is a very delicate catfish that feeds on epibioses. The fish requires very clean water with stable parameters and regular water renew. Otocinclus needs thickly planted tank with sufficient number of shelters and soft substrate. You should feed the fish with special tablets for catfish and vegetables.

But the biggest problem is that there is almost no algae that it feeds on in a newly planted tank.

However, if you are able to feed the fish additionally and keep the tank water ideally clean you may succeed in keeping it in a tank. Just there are less demanding species with alike behavior, for example, bristlenose pleco.

Koi fish

Koi fish can be most often seen in ponds, because this is the place where the fish will prosper. The thing is that Koi grows, as well as all carps do, to weigh about several kilos.The fish requires up to 400 liters for one species and this is more than even experienced aquarists can offer.

At that you may see Koi and goldfish next to each other on sale and quite often nobody warns the beginners that Koi is a pond fish.

Redtail catfish

It’s an interesting and in its own way nice catfish, that’s why the beginners often buy redtail catfish. Of course, sellers say that it is very enduring (and it’s true), grows well (you’d bet!), eats everything (especially small tank mates), but nobody says a word about the size the adult fish becomes.

Redtail catfish grows to weigh up to 80 kilos in the wild. In a tank, of course, it’s smaller but not much smaller. Again it should be kept in very large tanks with very large tank mates.

Iridescent shark

The Iridescent shark that we can most often see on…supermarket stalls. Actually iridescent shark is bred in South-West Asia on industrial scale to sale its fillet. And as you may guess they breed this fish definitely not because it is small or grows poorly.

It grows to be up to 1.5 long and it is incredibly gluttonous. When kept in a tank the fish is rather timid, it swims all around the tank when it gets scared and it ruins anything that gets in its way.

Iridescent shark is dumb, if you can apply this notion to a fish, and it can also pretend to be dead which is also confusing.

Giant snakehead

This is an active and very gluttonous predator as all snakeheads are. Provided with a proper tank capacity and good feed it may grow 10-15 cm a month. As all predators it eats everything that moves and fits into its mouth.

After the fish becomes 30-40 cm long its teeth become big enough to bite a piece of its tank mate of even the one of larger size. One fish requires from 300-400 liters of tank capacity. The tank should be elongated, wide and not very high. Tank water aeration isn’t necessary.

To breed the giant snakehead you’ll need a tank of about 1-2 tons capacity. The fish bright red coloring becomes grey-black with stains once the fish is 30-40 cm long. Also the giant snakehead is quite clever and fast.

Clown loach

This is a very beautiful and active fish which as a rule attracts the beginners. But it is too active and quite large. Here you may read more about clown Loach.

In the wild the fish grows to become 40-45 cm and about 20 cm in average in tanks. Clown loach shouldn’t be alone in a tank, so it should be from about 250 liters capacity for 3 fish. It’s not good to keep less than 3 clown loaches, since they’ll get blue and die.

The fish eats all kinds of snails – from ramshorn snail to large apple snail. Clown loach likes swimming fast in a tank and creating chaos in it. The fish swims only in bottom water layers. It may lie sidelong on a tank substrate when it sleeps.


And our list ends with not quite a fish, or more exactly – not a fish at all, but an amphibian. However, this never stops aquirists from buying it and keeping in a community tank.

Axolotl is an eastern tiger salamander (Ambystoma tigrinum) larva, the feature of which is that it can skip turning into its mature form and stay aquatic.

The tank water temperature shouldn’t be higher than 20°C degrees. No fish in a tank where axolotl inhabits! Either axolotl eats all the fish, or the fish tears axolotl gills – both isn’t good.

No snails in the tank too – axolotl may eat them and then feel unwell. The tank substrate should be large grained, so axolotl won’t eat it accidentally.

The required tank capacity for 1 species is 30-50 liters. You’ll need not very big tank with large tank bottom square and good filtration as well. It’s quite difficult or moreover unreal to make axolotl transform into its mature land form. It’s obvious, that even for an experienced aquarist it’s rather difficult to call all above listed requirements easy ones.