Salinity & Temperature Guide

Quick answer: salinity is set on the filtration network, not on the tank or the animal — you choose Freshwater, Brackish or Saltwater for the whole network. Temperature is set per aquarium in the Environment tab, and it only becomes adjustable once the Aquarium Staff Gate is powered. A community range check found 24°C or 25°C sits inside the preferred range of every saltwater species, which makes either a decent starting setpoint — still confirm each animal in the Zoopedia.

Salinity

Three options: Freshwater, Brackish, Saltwater.
Set it in Edit Filtration Network on the Water Filter.
One network carries one water type. If you want a freshwater tank next to a saltwater tank, they need separate filters and separate networks. Sharing a network means sharing a salinity.
Match species to the network through the Zoopedia. A saltwater species in freshwater is not a valid mix; neither is the reverse.
Brackish is a third, distinct state. Use it only when a species asks for it — "close enough to saltwater" is not a thing the game honours.

Temperature

Set in the Aquarium Info Panel → Environment tab. Lighting lives in the same tab.
Requires the Aquarium Staff Gate to be powered — an unpowered gate means no temperature control, however many heaters you think you have.
Each species has a preferred range in the Zoopedia. Outside that range, welfare drops.
Cost scales with the gap between the set temperature and the current weather temperature. Grouping species with similar needs in one tank keeps the bill down; a hot-climate tank in a cold map is the expensive version.

The 24–25°C shortcut

A community range check across the saltwater species found 24°C or 25°C inside every listed preferred range (grade C — one guide's check, not a Frontier statement, not something we tested in-game).
Use it as a starting setpoint for a saltwater tank, then verify each species in the Zoopedia before stocking.
If two species' ranges only partly overlap, set the temperature inside the overlap, not at one species' extreme.
It does not replace salinity. Temperature never changes water type, and salinity never changes the temperature range.

Worked example: starting a saltwater tank

Build the closed loop and place the Aquarium Staff Gate — Aquarium Build Guide.
Power the gate.
Place a Water Filter and connect it to the node on the gate via Edit Filtration Network.
Set salinity to Saltwater.
Set temperature to 24–25°C in the Environment tab.
Stock only saltwater species, checking each in the Zoopedia.
Add filtration as you add animals — demand rises with population.

Common mistakes

Mistake	Result
Set salinity on the filter but built a second tank on the same network	both tanks share one water type
Tried to set temperature before powering the gate	no temperature controls at all
Used 24–25°C for freshwater species without checking	possibly outside their range — check the Zoopedia
Assumed Brackish is "close enough" to saltwater	salinity mismatch, welfare drops
Set temperature at one species' extreme	tankmates at the other end of the range suffer

What is still unpublished

Per-species water type for every animal, exact preferred temperature windows in a single table, and any statement about how fast welfare decays outside range. The Zoopedia per species is the authority.

Sources

Frontier Player Guide — Aquariums (grade B: salinity chosen on the filtration network, temperature in the Environment tab, power requirement)
checkpointguides — Planet Zoo 2 aquarium guide and gamefieldguide — aquariums & aquatic animals (grade C: the 24–25°C saltwater range check)