I suppose you are really a bit too enthusiastic about their growth. In intensive culture for meat production they reach a weight of 1,5kg during 9months, but at this weight their are far away from 4feet.
Even in France Silurus glanis won´t grow to 10 feet. Just keep in mind that you can be in most countries very lucky to get a least one of 1,5m and for most people one in the 2m range is the fish of their life if the ever manage to catch one.
It is true, in some regions like Souther France and Italy they will much more regularly reach bigger sizes, and lengths of 2m and more are there not that rare, but still far away from being normal. The biggest french wels was 2,50 m and weighed 105kg, and that´s really exceptional. Even the world record is "only" 2,78m and this was one single and many decades old specimen among tens of thousands. So it is highly unlikely that your wels will ever reach a size close to 3m.
I don´t doubt that they will probably show massive growth which is better than in nature if you feed them enough and if they have warm water. But you should not overestimate this species and think that 10feet are normal. In fact the average in middle Europe is about 1,2-1,4m with few exceptions over 2m.