How can I get my fish to shoal?

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Why don't fish that shoal in nature swim in the same way when added to the aquarium?

Why don't fish that shoal in nature swim in the same way when added to the aquarium? Bob Mehen offers some advice to a reader who's having just such a problem with her tetras...
 

Q. I recently bought eight Penguin tetras for my community tank, which has been set up for six months. I've always liked them, particularly as in many of the pictures you see of these fish — and also at the shop I got them from — they seem to shoal really closely.

But since they went in my tank about a month ago they don't really shoal any more. Is this because I don't have enough of them — and should I get some more?

The tank is 90 x 38 x 38cm/36" x 15 x 15" and also contains 10 Neons, four Peppered corys, a pair of Neon rainbowfish and a pair of Kribs, which I bought last week. All the water tests are fine and the fish all feed really well and seem healthy, but I would really like to see the Penguins shoal.

Karen, email

A. Penguin tetras (Thayeria boehlkei) also sold under the name of 'Hockey stick tetra' due to the shape of their black bar marking, are always a popular community choice. They swim in a distinctive 'head up' position, sometimes in the tight groups you mention.

The reason that most fish shoal is for protection and in the sparsely decorated and brightly lit conditions often found in sales tanks the fish can be nervous and stick together in a close group. However once they are relocated into the more peaceful setting of our tanks they become more relaxed and confident so spread out in search of food. This is a good sign that you are giving your fish pleasant conditions in which to live but sadly means the tight shoaling that attracted you to them in the first place is no longer displayed.

Some people would suggest adding a larger species of fish, something like an Angelfish (Pterophyllum sp.) to make the Penguins more fearful and therefore likely to shoal closely, but personally I wouldn’t do so; why would you want to stress your fish unnecessarily? Also in most cases the small fish will soon become accustomed to their larger tank mate and realising it’s not a threat will return to their relaxed and dispersed swimming.

One possible solution is to increase the amount of flow in the tank. If you can get a steady, but gentle flow of water from one side of the tank to the other (perhaps with careful positioning of an external filters outflow) then the Penguins may hold a close shoal again as a way of conserving energy and effort.

A bigger shoal is a good idea anyway and will also prevent the fin nipping these fish are sometimes responsible for if kept in too small a number. But don’t be upset if they still won’t swim in a tight group — it’s a sign you are giving them a comfortable, stress-free home which should after all be the major objective for all aquarists.

Bob Mehen

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