It should be "are in this pool"

How many litres of water are there in this pool?

The contributors really need to include "The English sentence is unnatural or has an error" or "The 'correct' sentence is unnatural or has an error" among the possible boxes to check of on the report function.

"are" not "is" - the correct translation is: How many litres of water are in this pool?

Agree, there should be another report box for bad English grammar. Such as 'are' for plurals