There isn’t a single correct camera/flash setting here — it depends on the actual light levels and the look you want. A good approach is:

• Expose the aquarium background first so it keeps its natural dim look.
• Add off-camera flash for the model, rather than relying on candlelight alone.
• Use the softest light you can manage: ideally a large softbox just out of frame, or at least a diffused off-camera flash.
• Warm the flash if needed so it matches the candle feel.

Using only the candle usually means long exposures, high ISO, and possible motion blur. Supplemental flash is the practical way to keep the subject sharp while preserving the background.

If reflections from the aquarium glass are a problem, experiment with placing the flash at an angle to the glass and using a circular polarizer on the lens. This can help reduce direct reflections while still allowing the tank and water to be lit.

Also consider compositing two frames if needed: one exposed for the aquarium and one for the model. That can make balancing the scene much easier in a difficult low-light setup.