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We have a 20 gallon tank that has been running for about two months. About a month ago we added two diamond tetras and one died within a few days. Yesterday we added two more diamond tetras and a bottom feeder. One tetra became extremely aggressive, bumping and nipping the other two tetras. After some research, we added 3 more tetras to the tank and rearranged what we could to make the territory seem new. We also turned off the light in the tank overnight to put them in "sleep mode" hoping the aggressor would be fooled into thinking this was new territory and a nice school to join, but to the aggressive behavior has continued. The other 5 tetras are huddled in one corner and 2 or 3 of them won't eat. Is the only solution to remove the aggressor?

Hi, & welcome to Fish Profiles.

You could have a rogue tetra, or, more likely you are seeing the interaction between "oldtimers" vs the new kids on the block. Even in large schools, there is a pecking order, and each fish soon learns its place in the school.
Unless the one is a rogue, they should all become acclimated to each other.

These fish need to be in schools of (depending upon who you read) 5-8 fish. They need heavily planted tanks, and lower light. Be sure that you have a good plant light, one that puts out light in the 6700K range and that will provide around 2 watts of light per gallon. Low light plants, and some medium light demand plants will work just fine. Leave just a little "swimming room" in the center of the tank, shaped like a horseshoe, and plant heavily across the back and along the sides. Stem plants that grow to the surface and across it, will dim the light for the fish. Crypts, anubis, are great low light plants, and most of the stem plants will also work.

Your comment about leaving the light out for the night has
me concerned. You will need to provide at least 8 hours of darkness for the fish to rest. That includes leaving the "moon lights" off as well. If you put live plants in the tank, you should provide lights on for 8-10(max) hours. If you have moon lights, then leave them on for a brief evening period, and then turn all the lights off.
These fish swim in heavily planted, slow moving, streams, and the plants provide shelter from overhead preditors
such as wading, and flying birds. If you have no plants and just a few ornaments the fish will not swim about and will huddle in fear of predation.

I hope this helps...
Frank

-->>> The Confidence of Amateurs, is the Envy of Professionals <<<--

Thanks for the tips. We are very new to this. We had asked the fish store lady about adding live plants, but she thought the tank was a little too new for the plants to thrive. We have one large "plant" in that goes to the top of the tank and the aggressive fish goes back behind it and comes out to drive the rest of the fish up to the top of the tank away from the plant.

How long should I wait before I know if the tetra is a rogue?

Hi,
Boy, that is the $64,000 question. With just one plant,"he" is claiming it for his own and driving the others away from it. I would try some stem plants. Get a couple of bunches of them and unwrap the binder at their base. Then plant them, two or three stems per hole.

Another option is to place some "dither fish" in the tank. A small school of five fish such as "zebra's" will distract the problem fish, and also show the others that it is safe to swim out in the open. You have a small tank, so don't put any more fish in the tank as it will become over crowded. Your five or so diamonds, and five zebras should be just fine.

In a sense the store owner is correct, a tank that is only two months old will not have developed the nutrients in the gravel that is necessary to support a lot of plants.

You mention that you have one large plant that goes from the gravel to the surface. Is it an Amazon Sword? If so, they are "heavy root feeders" meaning that they get the bulk of their nutrients through a large root system in the gravel. Others, get their nutrients through their leaves, in other words from the nutrients floating in the water column vs nutrients trapped between the grains of gravel.

If it is a sword plant, they make "plant spikes" little cylinders of fertililzer that are pushed into the gravel around the plant (such as the sword) out far enough that you are not damaging the roots as you shove them under the surface of the gravel. They also make little fertilizer balls, about the size of a marble that can be buried in the gravel around the plant. In either case use only four and space them out around the plant in the four main compass points (N,S,E & W).

Frank

-->>> The Confidence of Amateurs, is the Envy of Professionals <<<--