Here’s Why Fish Tanks Shouldn’t be In Direct Sunlight

There is nothing like having a new fish tank. However, when the tank owners set them up at home, they can find that their location isn’t the ideal place for their tank. They go through getting everything right for their fish, and then it all falls to pieces when they sit their tank into position.

Problems occur when tanks are placed close to windows that receive lots of direct sunlight. In the beginning, everything can seem fine, and then tanks take on a murky appearance and fish begin to appear stressed. These leads to a nagging question.

Why should fish tanks never be in direct sunlight? Once you have a tank that sits in direct sunlight, algae growth can go into overdrive. The sunlight heats the aquarium water, and this alone can lead to many issues for your fish and your tank as a whole. The worst thing that can happen is fish may die from these temperature fluctuations because they are cold-blooded and unable to adjust their body temperatures.

If you see your tank is getting to much sun, it can take some effort to empty it and relocate it. However, it is better to understand why direct sunlight is such a problem. Read on, and you can learn why an aquarium should never sit in the line of direct sunlight.

What Happens When a Tank is in the Sun?

Fish breathe by passing water through their gills. When they do this, they take the oxygen from the water and breathe out carbon dioxide in the same manner as humans. During the day, any aquarium plants you have will absorb this carbon dioxide, and through photosynthesis, they replace it with more oxygen.

When you have too much light, as there would be in direct sunlight, there begins the growth of algae. Once this growth begins and you don’t do anything about it, then your tank water will take on a green appearance.

Once you have more experience, you find that tanks always have a certain amount of algae. It is nearly impossible to eradicate, so all you can do is to control it. You can see this when you have your light balanced as spots of algae can grow on rocks or the sides of your tank. These tiny amounts can be okay because some fish like to nibble at these.

The other side of this is it adds to the realism of your tank. If your tank is anywhere near a window that receives sunlight, you may only require a few hours of artificial light per day. In reality, you may be delivering artificial light, and as soon as the sun hits your tank, it is here the algae growth gets a boost.

What Causes Rapid Algae Growth?

Algae is like any other plant and requires the same basic things to grow. This is light, water, and nutrients. Once you have an excess of these, then algae will grow at a vast rate.

An aquarium needs to have water, yet the other two; you can do something to regulate the exposure. Here are factors that will go a long way to giving algae precisely what it requires to grow like crazy.

  • Keeping water which has a high nutrient level
  • Leaving water changes too far between
  • Overfeeding your fish
  • Artificial lights left on too long
  • Aquarium hit by direct sunlight

How to Avoid Algae Growth

Several things can help to keep your tank water clean and free from algae:

  • Reduce light: Keep your tank out of the direct sun and only use artificial lights for no more than 10 hours per day.
  • Don’t overfeed fish: When you give fish too much, most of this will sink to the bottom of the tank. Phosphates are released, which are good for plant growth.
  • Regular water changes: This is the best way to prevent large algae growth. Perform partial water changes of your tank for about 20% each week. This can help keep nutrient levels low.
  • Clean your tank: If you see algae growing on rocks or the sides of your tank. Scrape them clean and vacuum the tank gravel when you carry out water changes.
  • Use plenty of plants: Tank plants can help against algae growth. These plants absorb nutrients that would otherwise feed the algae.
  • Co-habit with algae eating fish: There are many breeds of algae-eating fish, so placing a couple of these in your tank can help to stop this scourge and save you lots of effort.

Sunlight and Aquarium Temperatures

Once you have a tank that sits in direct sunlight, it will heat up. The number of hours, the strength of the sun, and the size of your tank will determine the degree it does heat. A larger tank can have some buffer against overheating, yet they can still get warm enough to affect your fish.

Most of the fish you are likely to keep will have ideal ranges of temperature. This comes from out in the wild where temperatures do fluctuate within reason. However, they won’t suffer these temperatures for extended periods.

There are dramatic extremes fish can go to with their temperatures, yet you wouldn’t want to attempt this in case they are shocked and go belly up.

Waters in natural habitats can take a long time to warm or cool quickly. However, your tank doesn’t have the same luxury. Any rise or fall in external temperatures can make the water colder or in this case, hotter very quickly. These rapid swings harm fish more than anything does.

Because fish can’t adjust their temperatures (cold-blooded), their temperature follows the temperature of the surrounding waters. If it goes colder, they get colder. Here, in direct sunlight, they will become warmer and more uncomfortable. They will also become hyperactive or at least livelier.

If your water doesn’t drop too much, you may think this rise in temperature is harmless. However, these changes weaken the immune system of fish through them being stressed. Once you expose them to temperature stress, they are easy targets for parasites and diseases. Now they have a weak immune system, and they can easily die.

Tank owners may not be aware of is that beneficial bacteria also require a temperature range. This falls from 65 to 85-degree fahrenheit. If it rises above this to the ninety-five-degree point, these bacteria can die.

One side effect of this is that you can suffer from ammonia spikes in your aquarium. On the other hand, cooler temperatures can make your tank cycle slower.

Once the water heats up to a certain point, there is a chain of events that occurs. These are expedited once the temperature reaches or exceeds 90 degrees fahrenheit.

  1. Fish become more active and use more oxygen
  2. Warm water holds less oxygen
  3. Fish can quickly suffocate
  4. Beneficial bacteria need oxygen to break down ammonia
  5. With less oxygen, ammonia levels increase
  6. This displaces oxygen and burns the gills of your fish

Recommended Aquarium Temperatures

It is hard to say what the ideal temperatures are for individual tanks. There are too many factors; however, we can say what the ranges should be.

There are three temperature ranges, and these will vary depending on your fish.

  • Cold-water: Temperatures should remain below 68 degrees fahrenheit.
  • Tropical: Temperatures between 75 and 80 fahrenheit.

There is one additional class, and this is temperate fish. These often overlap into either of the above temperature ranges. Even saying this, temperate fish still have a preferred temperature range in which they can thrive.

When you begin looking at which fish to have in your tank, be sure to check the bodies of water they come from in their natural environment. If they come from large bodies of water, they are not as tolerant of swings in temperatures as fish that come from smaller bodies of water.


It is safe to say; no aquarium should be sitting in direct line of the sun. It can cause too many issues for your fish and your tank. If you have issues with water temperatures, even if your aquarium is in a shaded part of your home, this is a different matter altogether.

You can use fans to blow across the surface of your tank, or you can purchase aquarium heater controllers that allow you to heat or cool your tank as required.

However, even with these controllers, there is no reason to leave your tank sitting under the sun. Move it into the shade, and let your fish be happy.