Many thanks for your help Nays, that's an excellent link and told me what I need to know and what you said is correct - there is air leaking out and I suspect it's at the top of the inverted tank because as the water drops from this tank and the lower tank rises, it stops when the leak is underwater of the lower tank. That's my guess as to what is happening anyway. I'm going to have to remove the inverted tank and reseal it.
To answer your question, it's in an aquarium. My angelfish just love it and a couple have laid eggs in it a few times