it does seem too small, but the funny thing is when seeing it in person it didn't seem (at least to me) as small as it looks in the photo. It definitely needs to be bigger but I think the photo skews the perspective a bit.
elephant-trunk snakes are fully-aquatic but also very sluggish. A lower water depth probably suits them better because they then don't need to move much to breathe. I doubt they like bright light though. This picture (and I think Zooish's earlier one) are taken at an angle which I think is the reason it looks smaller than it really is. But yes I would prefer it much larger still!
2 metres?! That is HUGE! Heck, this tank would be extremely inadequate before they even reach 1.5 metres. It's like being stuck in a box that is only twice your size.