My aquarium is relative deep, 80cm (35"), and I only had one time when my eel climbed into the overflow. This was in the first week in the tank (we are now 1 year and counting since) and was possibly just an exploratory tour. My eel is very happy in his den under the rocks and rarely comes out completely, prefering to bob about in the current sniffing for possible tidbits. Apparently, once they have chosen a home, the remain true to it.
Clearly, these animals require a secure, enclosed area to feel at home. I doubt a bare bottom aquarium would ever make them really happy, even with PVC tubes. Current is important to them, as they always stick their noses out into the current to taste the water. My aquarium has a continuous current that runs along the bottom of the tank, which has always been the prefered hangout for my eel.
Another interesting point made in the German website was the gregarious nature of his group of eels. If a fish attempted to take the food from one eel, the others would join in to defend his morsel. Solidarity! Interestingly, it has been documented that eels will hunt in groups on the reef chasing out small fish for others to capture.