Thanks for the thanks for the inspiration...
This mermaid of yours needs to get done boy.. It's been way too long.
As for deep sea anglers, the deep sea is really a pretty empty place, even as it is there is just such little light, and there really is no scenery at 1000 feet in the open ocean.
Just having her in pure black would look good, but if I were to do an angler mermaid (bound to happen someday- I really like the mermaids!) I would likely have some deep sea favorites in there. Viper fishes, squids, etc.. Remember- deep sea fish are for the most part under 1' long, so for scale they would need to be either in front, or balanced out in some other way scene -wise.
Your mermaid looks as if she is angling, so putting something cool close to her might work, but what would a deep sea mermaid angle for? Maybe a drowning diver? A hand reaching out? Halloween is close- better stop thinking too much! Remember- mermaids are purely human-interactive fantasy- making her angle for a fish seems odd?
Good luck.
Craig