Hi and welcome to the forum
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How many gallons is the tank? Depending on how many gallons it is, you could have plenty of space for new fish, or no space, or even have to little space for your current ones.
You need to test the water for ammonia, nitrites and nitrates to see if the tank is in good shape for adding new fish, as if there are any unnoticed issues, they may get worse with the addition of new fish.
Also check the health of your current fish- if they are unwell, adding new healthy ones to the tank will not be good- if your olds one's died of something like water quality issues or deseases like whitespot or internal parasites, its best to sort out those problems completely first than run the risk of them coming back with the addition of new fish.
Now is also a good time to check out your tanks gear and habitat to see if it is suitable for new fish or is not ideal for your currents ones- things like not powerful enough filtration or unsuitable habitat like lack of hiding places or unsuitable substrate could be a problem.
If the tank is new and cycling, that needs to be looked into as well before making any purchases
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When you buy new fish, make sure you acclimatise them for at least 30mins, by floating the fish in their transport bag in the water and slowly letting in small quantities of your tank water into their bag from your tank to allow them to adjust to the water and temperature.
Check out this article on choosing healthy fish;
http/www.fishforums.net/index.php?showtopic=46271
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