You may test bulbs (LEDs) in a lighting fixture that does work first.
[This would demonstrate that the LED or LEDs work].
You may test your lighting fixture (tank) circuitry with a non-LED filament bulb or bulbs that do work first.
[This would demonstrate that the light fixture (tank) circuitry works].
If both those pre-checks work reliably, (every time, no tweaking/twisting/tapping), then the swapping of LEDs for bulbs, or swapping the test fixture for the lighting fixture under test, should work. If not, it may be a polarity problem, and swapping plus and minus wires may fix it, or more simply putting the batteries in reversed, (but not both - swapping wires and reversing batteries).
Some fixtures may not have two wires to swap, if the metallic housing is used as an electrical conductor.