I want to add more oxygen to main tank anyway ..
At present the filter (Fluval U2 plus) is positioned so that there is a lot of surface agitation as the water returns to the tank. I was hoping that there would have been a way to add an airline and airstone to this filter but I do not have the instructions and can't find anything on google.
For those not familiar with the filter.. there is a blue outlet running horizontal where the water returns, and there is a black pipe vertical above this where air enters..
I do know if I can't do it with my filter I'll need to purchase a separate airpump, a line, a non-return valve and an airstone attachment!
Also, I was purely just unlucky with the dealer - they were voted No1 aquatic store in the North of England by readers of "Practical Fish Keeping Magazine".