I have an Oase Biomaster 600 on my 300L aquarium which was set up nearly 3 months ago. It had been working well but recently keeps accumulating air And now I have to purge it twice a day by rocking it from side to side. It is frustrating as it reduces the flow. I cannot work out how the air is getting in there. I have checked all the pipe joints and they are tight. Has anyone else had this problem and any advice please.