The most important is that you have some space (few cm) behind tank. The rest is up to you. Experiment a little and leave the solution and position of light that you like the most. RGB LED with controler is a good option for backlight games...
You can use blurry (milky) foil. It will help to dissipate the light and will make shadows softer. Gradient is normal from light source where is the maximum brightness and coloration to outward it decreases
That back lighting looks very natural. A question for you have you ever tried a dimmer switch you could get some very interesting photos or a Video as in sun rising and sun settings.
"I am thinking about studio lighting Second hand to shoot better photos.
Read some old threads with valuable info but don't really know where I Should look for when buying this Second hand?"