Young SALs tend to stay near the surface since they're closer to the air. However, don't discount the notion that there may be something wrong with the tank. Is this a new setup? If not, how often are the gravel washes/water changes? And, how much for each cleaning? What's the temp of the tank?
The LFs mouth may look small but, don't get comfortable when working near its mouth. Their jaws expand wider than you think. And, their jaw strength is immense. Their dental plates can sever a finger so quickly that some victims of LF bites have reported that they didn't realize they'd lost part of their finger until seeing the lost digit.