What term describes organisms that maintain internal fluid concentrations equal to the saltwater environment they inhabit?

The correct term for organisms that maintain internal fluid concentrations equal to the saltwater environment they inhabit is "osmoconformer." Osmoconformers are typically found in marine environments where they actively match the osmotic pressure of their body fluids to that of the surrounding saltwater. This adaptation allows them to maintain cellular functions without expending significant energy on osmoregulation, which is the active process of regulating fluid and electrolyte balance.

In contrast, osmoregulators actively control their internal environment to maintain a consistent osmotic state, regardless of the external conditions. Homeotherms are organisms that maintain a stable internal body temperature, while ectotherms are those whose body temperature varies with the environment. These latter terms do not pertain to how organisms manage their internal salt concentration in relation to their environment, which is specifically what osmoconformers do.