OK..you said your tetra whisper is internal and your description did seem to match that since the water does go in the bottom area of them. Tetra makes both internal and HOB style filters...so I only have your description to go on. If it's the type I described that sits behind the tank with an intake tube hanging over the edge and down into the tank, that's a HOB. If the entire filter is inside the tank and it is suctioned cupped to the side...that's iternal. They are just two different styles of power filters, the HOB is typically preferred because it frees up space inside the tank.
Maybe it's not an undergravel filter then. Like I said, if it has a plate with holes in it at the bottom of the tank. I was picturing one of those little Hawkeye 1 gal kits they sell at Walmart, since you said it had a bubbling tube. If it is, those systems are crap. You would want to chuck the plate and tube and attach the airline to a small sponge filter. All a tank that small is good for is shrimp though.