The Xindi-Aquatic language was the very complex language used by the Xindi-Aquatics. It consisted of high or low pitch hums that were similar to those produced by Terran whales.
The language switched to SONAR when the past tense was used. (ENT: "The Council")
Hoshi Sato said that she thought the Xindi-Insectoid language was difficult while trying to translate an Aquatic message. (ENT: "Damage") Commander Dolim heard the language his entire life, but it never came easily to him. (ENT: "Countdown")