Hi I'm setting up a 17litre tank for my 3yr old birthday. I have no fish yet. I'm using a Rena super clean 40 that came with it but as soon as I plugged it in its making a horrible noise?! I think I've put it together right but instructions were poor. Is the whole unit supposed to be underwater or just the hose bit? Can the cable wire be underwater or is that a stupid question!? Help please