Very weird, I had ten tanks with cartos and 2 bcc tanks and they screwed into the hippo like they should and then this one would not go about a quarter turn without feeling that something was going to get damaged!! I just tried another new carto, it would not work but a third new one out of the pack crewed in like butter!! Tells me, the threading is just slightly irregular on these few I have that won't srew down....ummm....or do we contact vendor, I will ponder on this one.Yes I meant it won't go in at all, maybe for only 1/4 turn at most. I am ripping my hair off trying to determine if I got a defective Hippo v2 or if the tatroe bases are not strictly 7mm x .5 thread. I got two tatroe bases and neither will go into the Hippo v2 at all