Hmm... that's not how my little filter works. There's a tube that runs from the little outside motor to the bottom of the filter inside the tank. The tube fits into the bottom of the filter and there's no tube 'loose' in the water.
...This would be so much easier to work out of it was my main filter!
I think, looking at it again yesterday, in my case the water gets taken up through the filter via an opening underneath it. But if that's the case I'm not actually sure what the tube is for unless it's just to create the air flow. Bubbles come out of the top of the filter via another opening. It's a bit hard to describe really and I'd get a photo but I can't get at the right angles to show you.
Anyway, I think I've worked it out. If I can get away with only using one sucker to hold the filter onto the tank (which I see no reason why I couldn't) then I can slide the tights I got yesterday over the bottom of the filter covering the opening (and over the lower sucker). I'd need to cut a hole for the tube to fit through but I can easily make a draw-string type hole so I can then pull it tight again so there's no gap. And as far as I can tell that should work.
I don't need to cover the top of the filter, just the bottom where it sucks the water up? I don't need to keep the babies away from the bubbles? Although I can't see that's they'd be strong enough to get too close to them anyway.