Modest Skirt → Mermaid Tail

On a whim, I decide to try and make the fact she's a mermaid more apparent. I figure the simplest way to do that is to get her 'Modest Skirt' out of the way by replacing it with a 'Mermaid Tail.'

The frame of the window vanishes from behind Arma as it turns into an open archway, while her hair becomes just a little less tamed and the jewel vanishes from her tiara. Beyond this, her scaled leotard - corset? - remains the same, but the tiny frills of her skirt vanish. Surprisingly, and maybe a bit annoyingly, her hips are still clearly human and there's no sign of-- Oh. Her tail is only visible via the fin at its end, somewhere behind her.

I wonder at the comfort of however she's posed but I suppose I can't complain too much, given the change simply didn't materialize how I expected it to. Still, this is going very slowly.

What should I try next?

(summoned beast, sexy and slutty mermaid, emerald-green hair, braided hair, innocent expression, tilted head, one hand forward, showing her large ass, plate armor, mermaid tail, tiara, inside a castle hallway)