Keep in mind that the specular lights are reflections of the actual light source. The softboxes were the right choice, but they are positioned too far away and high. Bring then right up to the tank on each side, then move them back and forth (or forward and backward) while watching the reflections on the tank. You'll find the right location. Its very similar to lighting glass. Large light sources close to the object, and positioned so that the reflections (always at 90 degrees btw) are where you want them. Here is an example I took a while back for a client;
