I think you're overstocking. Way too many fish for that small a tank. It holds a total of 15 liters, for 21 fish... that's not quite ideal. I don't think having a cycled filter will do much, it's just too much load on a tank. While you may be able to keep them alive, they cannot expect to thrive. I guess it depends on the shape of your tank as well, more surface area equals to more fish you can keep in there. Still, it's a matter of having healthy happy fish or surviving, cramped fish that will not fulfil their potential in terms of growth or anything.

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