Prepare the hospital tank before you touch a net. Isolation done in the wrong order, with a fish chased around a bright tank and then dropped into cold, unfamiliar water, stresses the patient and the whole school. Done in the right order, the move takes a minute, the school barely registers it, and the sick tetra gets quiet water, close observation and separation from tankmates while its condition is assessed.
Setting Up the Hospital Tank First
Fill the hospital tank with water taken from the main tank, because matched water removes the temperature and chemistry shock that a fresh fill would cause. Add a gentle filter, ideally one carrying established media from the main system where your setup allows, and a heater set to the main tank's temperature, verified with a thermometer rather than the heater dial. Add one simple hide, such as a clean plant pot or a piece of decor from the main tank, and keep the layout bare otherwise so you can see the fish and its waste clearly. Dim the room light or cover part of the tank with a cloth. Let the setup run and settle while you watch the main tank and choose your moment.
Moving the Tetra With Minimal Chase
Work in low light, ideally when the school is calm rather than feeding. Use a soft, fine net or guide the tetra gently into a clear container submerged in the tank, because lifting a small fish in water inside a container avoids air exposure and net abrasion. Keep the chase under a minute: if the tetra evades you repeatedly, pause, let the school settle, and try again rather than escalating. Move the fish directly into the hospital tank and withdraw quietly. Avoid moving other fish, rearranging decor or doing a water change at the same time, because stacking disturbances is what unsettles the school. The remaining tetras should resume normal schooling within a short period.
Observation, Feeding and Water Checks
Feed the isolated tetra sparingly, because uneaten food degrades water fast in a small tank with a light filter load. Test the hospital tank water regularly for the parameters you normally monitor, and test the main tank too, because a problem in one fish often reflects conditions the whole school shares. Record appetite, swimming position, breathing rate, fin condition and any change in the marks or behaviour that prompted isolation, with times attached. Photograph the fish daily if you can do so without flash or tapping the glass. Do not add medication on your own judgement: treatment decisions for fish belong with an aquatic vet or an experienced aquatic service, who will want exactly this observation record.
Deciding About Return, With Advice
Ask the aquatic vet or service what recovery should look like and what signs would mean the tetra can return, because return criteria depend on the cause, which you may not know. Keep the school under observation for at least the same period, watching for similar signs in tankmates. When return is approved, match the hospital water to the main tank gradually and reintroduce the tetra in low light, watching the school's response for chasing or exclusion. Keep the hospital tank running until you are confident no second fish needs it. Your tank tests, observation notes and the advice you received belong in the tetra's KinsLe Pet Passport, so the next episode starts from a record rather than a recollection.