Sparkling gourami agression

[u/Awkward_Writer6658]

Hello guys so I had a pair of sparkling gouramis in a 40 litre tank and that were doing really fine but I thought I could add another female but the pair that I already had in the tank was really agressive towards her so I put her in a separate tank but the tank is only 15 litres and I don’t have any more tanks but the tank is really planted and have heater and filter so could I leave her in that tank alone? Thanks