# Water temperature

_Why every fish has a comfortable temperature, what the heater does, and how to set it for a mixed aquarium._

## What the temperature means

Every aquarium stands in a room at 22 °C, and its water drifts toward that unless a heater keeps it warmer. Every fish has a band of water it's comfortable in, shown on its card and on its [Reef Journal page](https://fishies.lol/wiki/species).

The gauge under your level and coins shows the temperature next to the [water quality](https://fishies.lol/wiki/mechanics/water). Open it to see where the water is heading, how many fish are comfortable there, and to set the thermostat.

## Comfortable bands

Every species belongs to one of four groups:

- Reef fish · 24 to 28 °C. Most of the catalogue, and nearly every fish until the lagoon fish arrive.
- Lagoon fish · 27 to 31 °C, like the Parrotfish. Warm shallows and open tropical water.
- Temperate fish · 17 to 22 °C, like the Knifejaw. Happy at room temperature, never in a heated reef.
- Hardy fish · 20 to 28 °C: the estuary wanderers and the Bristlenose Pleco. At home almost anywhere.

## What the wrong temperature does

Inside its band a fish is not affected at all. Outside it:

- Growth slows, down to 60% of its normal speed at 4 °C or more outside the band.
- Up to 2 °C outside, [gold quality](https://fishies.lol/wiki/mechanics/quality) is out of reach; further out, silver is too.
- Temperature never harms a fish's health and never kills it.

## Heaters

Every aquarium gets a Glass Heater, a [gift](https://fishies.lol/wiki/mechanics/next-goal) that arrives with its first fish that reads temperature and keeps a reef comfortable. The Equipment tab of [Build & Buy](https://fishies.lol/wiki/mechanics/shop) sells 3 heaters: the Titanium Heater from level 30 reaches 29 °C for lagoon fish, and the Controller Heater from level 50 reaches 31 °C.

A cheaper heater lets the water swing a little either side of its setting: ± 1 °C for the Glass Heater, ± 0.5 °C for the Titanium Heater, and none for the Controller Heater. A fish counts as comfortable only when the whole swing stays inside its band. A better heater trades half of the old one in, like all [equipment](https://fishies.lol/wiki/equipment).

## Setting the thermostat

Open the gauge, turn the thermostat with − and +, and press Set. It's free. The water warms toward the new setting at the heater's own speed, or cools by 0.5 °C an hour, and the gauge shows how long that takes. Fish aren't flagged while the water is on its way.

A heater can only warm: set it to Off and the water stays at room temperature, 22 °C. The gauge suggests the setting that suits the most of your fish.

## Mixed aquariums

Groups that don't share a band can't all be comfortable in one aquarium, so mixing them is a choice:

- Reef and lagoon fish share 27 to 28 °C, but only the Controller Heater holds the water tightly enough for both.
- Temperate and reef fish share nothing: keep them in [separate aquariums](https://fishies.lol/wiki/mechanics/aquariums), or accept slower growth for one group.
- Hardy fish get along with anyone.

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In the game: https://fishies.lol/wiki/mechanics/temperature · Fishies 0.8.23

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