Daily Questions

Daily Question: How long do I need to leave my aquarium light on each day? I don’t have plants, but the room is pretty dark, I’ve heard fish don’t need light.

Between 6 and 8 hours. If the room is really dark then maybe more.

Fish need a day-night cycle, because like us they sleep. Also, like us melatonin is an important factor in sleep for fish. A single injection of melatonin in Wrasse put them into a sleep-like state.

Now what do we mean by sleep? As I’m writing this one of my catfish is sitting in the flow of the filter. He’s not reacted to the other fish around him for at least a couple of hours, his fins are moving just enough to keep him in the same position. Due to having no eyelids his eyes are obviously open, but it’s clear that he’s in a state of low activity. That’s what we define as being sleep. They’re quiet and they have low arousal states. Some fish will burrow into the substrate, some will wedge themselves into cracks and crevices. Some will sleep on the tank floor, others will seem to hover in midwater. Interestingly tank bred Guppys will often sleep on the tank floor, but in the wild this is an unlikely behaviour. Is this due to the larger finnage, or is sleeping deeply on the floor preferred but just not as possible in the wild?

This all means that we need to mimic the day-night cycles that they would see in the wild.

Some lights have a dawn and dusk setting and can alter their output to decrease particular colours. This is a better, if a lot more expensive, option. I’m of the opinion that many species of fish also need full spectrum lighting, including the UV end of the range. But this isn’t something as well evidenced as it is with reptiles.

So what if you don’t have a fancy light? A simple timer can mean that you have the peace of mind that the light is coming on and off at the same time each day. I run my lights so that they’re on from mid-afternoon into the evening. These are the times of the day that I want to sit and watch my fish.

That’s the other thing. Your enjoyment. With no light how are you going to watch your fish?

The more light you have the more chance you has for algae. I would look at the nitrates and other waste products in your water before I worry about reducing the light cycle.

Further reading

Keene A.C., Appelbaum L. 2019 Sleep in Fish Models in Handbook of Sleep Research Ed. Dringenberg H.C. Chapter 24 p. 363-374


Leung, L.C., Wang, G.X., Madelaine, R. et al. 2019 Neural signatures of sleep in zebrafsh. Nature 571, 198–204 (2019)


Hur S. et. al 2012 Fish sleeping under sandy bottom: Interplay of melatonin and clock genes General and Comparative Endocrinology Volume 177, Issue 1, Pages 37-45


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Ruth McDonald

Sailed twice around the world, started my acedemic career as an archaeologist and somehow ended up lecturing on science and researching fish.

Tropical Fish Keeping UK