Aggressive rainbow shark

[FairyCodMother]

Hi everyone, I got a 15 gallon tank around June and it has a rainbow shark, two dwarf gourami and three black widow tetras ( this is what the staff member recommended). The rainbow shark hasn’t been aggressive until now but has started biting and chasing the other fish. I know I need to upgrade the tank size but that would be difficult at the moment. How long could I wait without it being cruel to the fish?
Thanks

[charliebankston]

Not sure how fast those Rnbw grow, but they are very territorial. Once it gets old enough to start aggression, it won't stop. And like you said, it'll be too big for a 15g. Id try to return or rehome it soon