Temperature: The Metabolic Controller

Unlike mammals, fish cannot regulate their own body temperature. They are the same temperature as the water around them.

This means Water Temperature = Metabolic Rate.

Tropical vs. Coldwater

Tropical Fish (74°F (23°C) - 82°F)

• Examples: Bettas, Angelfish, Tetras.
• Metabolism: High. They eat more, grow faster, and have shorter lifespans.
• Immunity: Their immune system works best at these temps. If kept too cold (< 70°F), they become sluggish and prone to Ich/Fungus.

Coldwater Fish (50°F (10°C) - 72°F)

• Examples: Goldfish, White Cloud Minnows, Doiio Loaches.
• Metabolism: Lower. They digest food slowly.
• Danger: If kept too hot (> 80°F), their metabolism speeds up to a point their organs cannot handle, shortening their lifespan drastically.

Stability

Just like pH, Stability > Exact Number. A daily swing of 2-3°F (-16°C) is natural (day/night cycle). A sudden swing of 10°F (-12°C) (e.g., doing a water change with cold tap water) causes Thermal Shock.

• Symptoms: Shimmying, lying on the bottom, immediate death.

Heater Failure: The #1 Hardware Risk

Heaters will fail. It is not a matter of if, but when.

• Stuck OFF: Fish get cold. Usually survivable if caught in a day.
• Stuck ON: Fish get boiled. Catastrophic.

Safety Tips:

1. Undersize: Use two smaller heaters instead of one big one. If one sticks ON, it isn't powerful enough to boil the tank quickly.
2. Controller: Use an external temperature controller (like Inkbird). It cuts power to the heater if the temp gets too high.
3. Check the Light: Look at the heater light daily to ensure it's behaving correctly.