Before you release any animals, it is vital that the animal you release should be that of your local species otherwise it will be detrimental to the ecological system in your country. Please take note as this is very important.
I never liked the idea of releasing birds especially in Thailand, firstly I feel like I’m indirectly supporting the business of the stall holder however in a few circumstances I did it because I saw many of the birds cooped up in small cages and were extremely stressed to the extent that some of them died and some were at the verge of death due to the extremity of the environment that they were in. I released two cages of birds which amounted to about over a hundred of them. The moment they were released you could hear the joy of freedom, they were happily chirping away. Can you imagine that if you were one of them and only a few were chosen to be released but not you, how would you have felt? The longer they are inside the cage the higher the chances of them dying from stress. I’m not in a position to say that releasing them is a good idea but I think you should act based on the situation as this can be a very contradicting and subjective topic.
It is said that when you release fishes or animals that are meant to be slaughtered as food it is also a good way to make merits for yourself. I personally think that when we do such things we should not be thinking about the idea of “just making merits” for ourselves. We should do what we should because we have a sense of compassion not because we’re after merits. On the other hand my Dharma teacher said that when we are suffering from a certain illness, releasing fishes back to their natural environment is one way for lightening our Karma, it can also help reduce our sufferings. Before releasing the fishes we should make a prayer for ourselves so that our sufferings will be lightened.