What you are referring to is the reflective quality of light. Put the fish in front...in close proximity to color decorations and you will get those hues blending with the original coloration of the fish. A darker tank will not change the color of any fish. What it will do is either dampen or enhance the light reflected back on the subject giving the perception of a change in color. I've seen this with hundreds of fish. On the photos above I used three NIkon SB900 flash units. Two above and one below.
For anyone interested in pulling more color and detail from your photos, beyond the lighting setup and camera settings (ISO 100, 1/320th @F32, top front light +1/3 power, top back light -1/3, below -1 1/2) the single best post production technique you can use is by adding two adjustment layers...Levels and Curves. Levels will allow you enhance shadows, midtones and highlights. Curves (most often used is linear curve which adjust equally) enhances the contrast. The rest of the photo is the fish. It's a given that the water is crisp, the glass is clean and any filters shut of temporarily to avid minute debris and bubbles...a real PITA when you are throwing the amount of light and getting the resulting detail.
