The leak was between the PVC elbow and the flange under the tank.
Elbow is 4" and the flange 3"... The other tank I did myself and no leak there. But this one, my friend did to fast and there where some holes in there.
Solved it with Silicone.
There's still leaks around where the two antenna's are screwed together, but waiting a bit to see if that settles with dirt.
If not, silicone again.
Water level at the 2nd drain is now at 19.5cm, after 3 hours of filling today.
Guess I might get another 2-3 cm today but that's it.
Nowhere close to 50-70% full, but alas, tap is not giving it faster.
Filling up very smooth and nice, no big folds or anything.
Amazing amount of space, I'm sure Mr.T. will go nuts when he starts sniffing around.