Can Fish Tanks be in Direct Sunlight?

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Unlike other pets, fish rely on their owners when it comes to oxygen and their tank environments. It’s therefore our responsibility to make sure everything is just right in their tanks. With summer rolling right around the corner, some fish owners are wondering if they have to make any changes to it their fish tanks.

Have you considered leaving your fish in the sunlight to enjoy the summer sun? Many experts don’t recommend this. Sunlight can damage a tank’s ecosystem by creating excessive algae, increasing the water temperature and causing stress for the fish. Continue reading for a detailed explanation with extra sources concerning how sunlight can be bad for your underwater pets.


Excessive algae releases harmful toxins into the water and reduces oxygen levels in the tank. Besides out-of-control algae being a danger to the fish, excess algae would also mean you would need to clean the tank more frequently than you already do. That’s a lot of extra cleaning!

Algae growth is something you need to be careful about. Excessive algae can also be caused by overfeeding your fish or leaving the aquarium lights on. Some things that can help with algae control are using natural plants and maintaining a regular cleaning schedule.

Overall Health and Stress Levels

Besides algae build-up, tanks should not be left in the sunlight because it can affect your fishes’ health and stress levels. Don’t forget that fish can get stressed just like you!

There are signs to look out for if you think your fish are stressed. According to, some ways that you can tell if a fish is stressed are

  • Fish staying near the surface gasping for breath, indicating that it has trouble getting enough oxygen (the concentration of dissolved oxygen is highest near the water’s surface). Possible causes include low oxygen concentration due to poor water circulation, toxins that have damaged its gills, and high ammonia or nitrite levels.
  • Fish won’t eat, or doesn’t eat as aggressively as in past.

It is a little harder to understand fish than it is with other pets. They don’t have wagging tails like dogs or the audible sounds that cats make. Observe your fish to know if they are stressed so you know what to fix in their environment.

Have you ever forgotten to bring sunscreen to the beach? It’s painful when you get sunburned! Remember this when it comes to your fish as well. In some situations, sunlight has even been reported to cause sunburn in fish!

“Although water generally provides a very good barrier against most wavelengths of ultra-violet light – the sunburn culprit – it is known that middle and long U-V wavelengths can penetrate water for a few centimetres, particularly in water of high clarity.” (University of Guelph)

Temperature Changes in Water

Think of it this way: If you have a fever, or are in a cold room without a jacket for a long time, you start to feel uncomfortable. Our bodies become accustomed to certain temperatures in our environment and it feels stressful not to have a coat or cold water when we need it. Fish feel the same if their water warms up too much.

The rising temperature also reduces the amount of oxygen in tank water. The more fish you have, the more important it is for you to give them enough water and maintain the correct temperatures. Keep a thermometer on hand to measure changes in your tank’s temperature.

According to Tropical Fish Magazine, a general temperature range for fish tanks is 70-80 degrees Fahrenheit, or 25-27 degrees Celsius. This number depends on the type of fish you own, among other factors. Be sure to do your research before buying a new fish!

Related Questions:

How do I know what lighting source is ideal for my fish?

Check out the links down below!

What happens if fish don’t get enough light?

Fish that don’t have enough light may become lethargic or may stop eating. Lighting can be provided from an artificial light or even the light from the room to stop this from happening.

See Also:
Can You Put Aloe Vera in a Fish Tank?
Can You Put too Much Water Conditioner in a Fish Tank?
Can You Put Decorative Sand in a Fish Tank?
Can a Fish Tank Cause Allergies? (Explained)

For more information, check out the links below:

Algae growth and prevention

Sunburn in fish

Correct lighting for your fishtank

Temperature and stress levels