I currently have a GloFish tetra tank with a black background so my led lights bounce and light everything else. It's for my 4 year old, and he's really into pokemon. I'm changing it for Christmas and doing pokemon theme (yes I'm sealing the figurines with the paint), but he likes "Alolan" pokemon (basically Hawaiian pokemon), so they're in the ocean and on the beach a lot. Question being, an ocean background would help the theme, but would it soak up my led light so my decoration and figurines wouldn't be as bright? I want the pokemon themselves to really shine with my expensive lighting. Black or ocean background?

A lot of people swear by black or blue backgrounds for enhancing the colors of their fish, and for making the rest of the aquascape stand out as well. Personally I like some variety though, I wouldn't want to always keep black or blue backgrounds on all my tanks. I think the ocean background sounds fine and if it turns out that you prefer the other background you could always switch it back later.

Btw I love Pokemon too! I'm currently playing Pokemon Sun. You should post pictures when you're finished doing the new theme!