Hi Dada,
In South Africa we have sharp-toothed catfish, which are very similar to wels, in the last few years I have been very sucessfull with large black flies , like a giant leech. When they are feeding close to the surface I use a team of flies, consisting of a big popper that is conected to a #1/0 black leech ( I use 40 cm of 25kg Maxima green - since they are not at all leader shy) this works very well since the catfish have very small eyes and rely on sound/vibrations to locate their pray.
The technique is to locate fish close to the surface then to cast as close as possible to them, the retrieve is usually hard pulls - to make the popper 'pop'. In my experience smaller fish - up to 5kg take the leech , the bigger oned tend to hit the popper.
Hope this works for wels... tight lines
