Hi Norbet - OK - I see the problem straight away!!
Plumbing 101for aquariums!
In fact, I am going to get our technical guy to put something up on our DD FAQ page about this to help stop anyone else from being caught out!
Aquarium plumbing and home plumbing (very confusingly!) are 100% NOT compatible!!
For aquarium use, you want to use 32mm pressure pipe (grey colour) 32mm pipe from B&Q and the like ( usually white or black) whether push fit or solvent weld will NOT be the correct diameter!! See below and website here http://www.hendersons.co.uk/pipework2/page2.html
The correct type of pipe can be purchased from any good aquarium shops .
Hope this helps and I wish you would have come to us sooner!!!
The specification highlighted within the green circle is correct , the specification in the red circle is incorrect.