Is the issue here one of grammar?
In the plural possessive, for nouns that do not end in "s", one would add an apostrophe and "s". For example "The three children's clothing was scattered about".
Conversely, for nouns whose plural form ends in "s", it is appropriate to add just the apostrophe...
For example "those sailors' lives were in danger".
In the singular, "that sailor's life...."
Accordingly, "fluids' levels" is the appropriate plural possessive...