I would like to ask for some inputs on how I can achieve similar low-key results for a fish: 
[![a fish](https://i.stack.imgur.com/YRpDA.jpg)](https://i.stack.imgur.com/YRpDA.jpg)

Current setup:
[![Setup](https://i.imgur.com/eGwsFJW.jpg)](https://i.imgur.com/eGwsFJW.jpg)

1. Small aquarium 30 x 15 x 15 cms (LBH)
2. The sides and the back of the aquarium are covered by a black muslin cloth.
3. The LED no longer has cardboard roll on it. I was using this to cut reflections, but it really reduced my working space. The LED helps me focus and provides continuous light.

Question:
How do I implement the **inverse square law** here given the small space? I don't understand how to bring the flash closer to the fish than to the background.

Thank you for reading the complete post.

Goal:

1. Perfectly and FULLY illuminated subject with no part lost to
shadows.
2. Pitch-black background.