Lighting is both science and art. If you get the chance consider going by a high end clothing store and as you walk through ask yourself what your eye is guided to and why. There are generally two types of light in that arena and they're referred to as ambient and task. Examples of how they're used in your home might be that an overhead ceiling mounted light that brightens up the entire room is simply ambient lighting. It's lighting up everything equally well. Task lighting would be the reading lamp at the end of the couch.
When you walk through that high end clothing store you're going to be walking through an ambient light level of somewhere around 30fc at about 36" above the finished floor but there will be much higher light levels that are often a different color as well and they're highlighting special displays in that room. Those displays are what you see first.
In your case you are likely attempting to use two identical fixtures intended for ambient lighting and you want to get some task lighting (or highlight certain objects) with them as well. That's not going to be easy and the best you'll probably find will be to orient them at an angle such that there's a greater implied depth. If you want to go next level you'll need a second type of light and it likely should be a different kelvin rating. If you're unfamiliar w/ what that means I might be able to help point you in the right direction if there are markings on your current lights to indicate color, color temperature, or degrees kelvin.
If you have an incandescent desk lamp in the house or something similar try temporarily placing it on top of the tank. Depending on how much different it is from the couple fixtures you already have up there you may see how highlighting, depth, light level and light color all play a big role in finishing a display.