Leaking water at the Aquamist 2D HSV valve

[mx5]

I installed the Aquamist 2D this last weekend.
The first day everything was ok. But then the pump started coming on even during at idle when the W/I was not used. I suspected a leak and found it - it is leaking water at one of the tube quick fittings at the HSV valve. It was just dripping on Sunday evening, but this morning I noticed that it even squirted a bit.
The leak at the fitting is not at the side where the 4mm tubing goes in it, but on the other side - where it bolts to the HSV.

I haven't tried fixing the leak yet, but when installed the system I tightened the fitting well, but by bare hands only. Is this the problem? Or any tips about sealing it?