I have an aquarium with some species (guppy, black tetras, neons) that have all grown up. Now I need to introduce new fishes, from the same species, but they are much smaller.
How can I prevent attacks, so that older fish don't bite their tails?
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In most cases, just properly acclimating and then releasing them is fine, this is assuming you don't have a quarantine tank setup in which you would first quarantine until you are certain they are disease and parasite free and then acclimate them to the display tank.
It is possible for some aggressions to occur and this greatly depends on the fish species. Often times, fish must learn where they fit in the hierarchy and unfortunately this can involve aggression. Small fish like the ones you're speaking about might chase and nip initially, but it is usually temporary and the new fish will join the existing ones in as little as a few minutes but some may take a few days or weeks to be comfortable.
With that being said, there are some situations where fish should be incrementally introduced to avoid that aggression because the aggressor will end up stressing or actually killing the new addition. This is typically done by isolating the new fish or dividing the tank with a clear partition so the different fish can grow accustomed to each other before they are physically introduced. Maroon or clarki clownfish in marine tanks are one example that can greatly benefit from this. Certain cichlids in freshwater tanks are another that can benefit from initial isolation to reduce aggression.
Also, in most cases, introducing very small fish to larger ones of the same species is a bad idea if the additions are small enough to be eaten. Fish rarely differentiate between a conspecific and a meal, so if they can eat what you add to the tank, they might just do that. Bottom line here is don't add juvenile or very small fish to a tank containing fish large enough to eat the new ones. They don't care if you're adding their cousin, they just see small fish as a meal...
In the case of this fish you're referring to, all of this should be completely unnecessary and if you are comfortable introducing them without quarantine, I would follow your normal acclimation procedures and release them with the existing fish in the tank. Dimming the lights can help the fish adjust and reduces the activity of the fish in the tank which can reduce chasing and nipping. The next recommendation is debatable because I routinely see advice for the opposite, but I have much better luck in almost every situation, feeding soon after adding new fish. This reduces stress on the fish that were just added and can immediately make the new fish feel more at home. Less stress = less opportunity for illness.