Colored Bathwater

Colored Bathwater is bathwater that is shown to be a different color rather than the typical colors of clear, blue or even Green. In SciFi environments or on other planets, the water (or whatever a person sulks in) can take on a reddish, pink or lavender coloring. Any unknown liquid that is an abnormal color that a character ends up bathing it goes her.

Najica Blitz Tactis
Humalites in NBT usually transform the water they bathe in to some kind of Nutrition/Rejuvenation liquid which (in the Anime) has a deep pink colour.

To Love Ru
School nurse Ryouko Mikado (who is an Alien herself) is shown bathing in a hot spring that seems to have a red colour, so it's very likely that it's located on another planet.

Taking into account that it was stated that Aliens are physically more enduring than Earthlings, it's also possible that it's a lava pit instead.