Welcome to AC!
While I'm not familiar with that particular brand of tank, the installation manual shows that there is supposed to be 3 o-rings on the intake stem and 3 o-rings on the output stem and if they are missing or were pinched when they were installed, this could be why it is leaking.
How snug did you tighten the ring nuts on the underside of the stems that hold them in position? The instructions say to just turn hand tight and not use any tools to avoid damaging the factory installed special fittings on tank openings.
You could try hand tightening the ring nuts a bit more but over tightening them may cause a worse leak.
If that doesn't fix the problem, you will need to empty the aquarium of all the water, remove the filter (if already attached), remove the ring nuts and the intake and output stems and check that the o-rings are not damaged.
Keep us posted on if this fixes the problem.