Oh it's actually quite small for the danios. They swim a lot and need a lot of space. And they do better in groups not less than 6. But I guess you can't add anymore, but this mainly depends on your filtration or if your tank is planted.
I suggest replacing the platies with more danios, but it's just a suggestion
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I asked the place I got them from and they said should be ok in the size tank as there bred in captivity but I'll see what happens looking to get a bigger tank soon