Tank Lights - How The Way We Use Them Affects Fish

When turning tank lights on and off, we must bear in mind the needs of the fish. We need to remember that fish's eyes do not react to light in the same way as our own. And we need to remember that fish are 'programmed' by nature to expect certain things in their environment.



Fish have eyes which are similar to ours. We both have a cornea, iris, retina etc. We both have rod cells in the retina which allow us to see in dim light. We both have cone cells in the retina which allow us to see colour. The main difference between us and fish is the way we control the amount of light getting into the eye.

We have eyelids we can close; except for a very tiny number of fish, they have no eyelids.
Our pupils can get bigger or smaller to allow more or less light into the eye, fish's pupils cannot change.
Fish's rod cells can sink into the retina in bright light and the cones can come nearer the surface in dim light. Our pupils can change size very quickly but it takes about 30 minutes for the cells in the fish's retina to move.

Fish cannot adapt quickly to changes in the amount of light. We should not do anything to the tank for at least 30 mintues after the tank lights come on – we should not feed the fish, do a water change, rearrange decor etc during this time.

And we should not turn the tank lights on or off in a dark room. Doing this stresses the fish, and we know that stressed fish have a lower imune system so they get sick more easily.
The room should have some lighting, either daylight or the room's light, for at least half an hour before the tank lights come on, and at least half an hour after the tank lights turn off. An hour is even better.



Humans and fish, like most animals, have a circadian rhythm or "internal body clock". Upsetting this circadian rhythm causes an imbalance which has to be reset – this is the cause of jet lag. The circadian rhythm is controlled by light and dark. In humans, our eyes are the main way we detect light and dark, though many of our other cells have a sensitivity to light. In fish, the sensitivity of the other cells is very high.
It has already been mentioned that the cells in fish's retinas move according to the amount of light. But these cells move in anticipation of changes in light. Fish "know" when dawn and dusk are coming and their eyes start to adjust in readiness. In nature, the time of dawn and dusk is very predictable, especially in the tropics where most of our fish come from. But our tank lights are not predictable to the fish if we just turn them on and off to suit us. We should have the lights turn on and off at the same time every day because the fish's circadian rhythm "expects" this.

Fish also have a need for a period of total darkness in every 24 hours. No tank lights, no room lights on, no daylight. In the regions our fish live in the wild, day and night are just about the same length – 10 to 12 hours total dark, 10 to 12 hours bright light, with dawn and dusk of half light between.
Tank lights do not need to be on for the same duration as this. The number of hours total darkness and bright light can be shorter or longer than in the wild – to the fish this just seems like a longer or shorter dawn and dusk. But they must have some hours of total dark and bright light in every 24 hours, with periods of dimmer light between. These hours of total dark and bright light should be the same length and at the same time of day, every day of the year.

I think in general our tank lights are too bright for the preferences of most fish. We only need bright lighting for plant growth. In many habitats (jungle streams, or anywhere with less than crystal clear water, etc) lighting isn’t all that bright.

I agree, floating plants or some other cover are necessary for so many species. I didn't include this aspect as I wanted to explain how the way we turn lights on and off affect fish, and how we can use the lights better to fit with fish anatomy and instincts. I've modifed the title to reflect this.

Those of us who have lights that are either full on or off have to provide another way of going from dark to full tank lights and vice versa. But those of us who have do lights that can be dimmed can use a dimming feature instead of opening curtains or having the room light on.

I've never liked the idea of UV lights anywhere, even shining out at me from a fish tank. Maybe I'm over sensitive but I've had skin cancer and now that I've got two 'new eyes' due to cataract surgery I don't want to risk the effects of UV light on my retinas  Did you know that yellow cataracts protect the inside of the eye from UV?

The Biorb MCR lighting has a 24 hour "cycle" which supposedly mimics natural day/night light inc. sunrise and set?!

However i have noticed that the "moonlight" is dim blue light - this doesn't seem quite right to me as it isn't fully dark but i suppose moonlight isn't complete darkness either...

I have been turning the lights off at night and back on in the morning, thoughts?

This was a very interesting article thank you it does bring a new thought process to me as I prepare to set up my new tank because I have always just used the on at dawn and off at dusk procedure. I also wonder wether I should change my lamp or at least ensure this one is not too bright.

I will be looking into automated lighting or how I appropriately do it manually where by I’m not late for work 

There are nights when there is virtually no moonlight - new moon - and clouds often obscure the moon. Many of our fish come from streams with a lot of overhanging vegetation, which will block moonlight more than sunlight.

Do the lights get brighter then dimmer to simulate dawn and dusk? Would there be any way to set them so that they are off during the night but turn on to the 'dawn' phase using a timer?

Hi Sue, yes they do get brighter/dimmer to simulate a dawn/dusk however they stay on at night time - reasonably dim blue light... doesn't seem ideal to me, i don't believe there is a way to make it turn off unfortunately.

I might link to it to a smart plug so it turns on and off itself...

My current tank came with fluorescent tubes, but the tube ends were not wired into the hood. They were fixed to the hood with clips and each tube had end caps connected by wire to a dual ballast outside the tank. The ballast only allowed the tubes to be on and off at the same time, so I used a single timer for this set up.
When I changed to LEDs the tubes clipped into the lid the same way, but now each tube had its own controller. So I just bought a second timer and had one for each tube so I can have them on a slightly different times so there is half an hour at each end with just one tube lit to give a sort of primitive dawn and dusk.

Since your tank has just one tube, a timer is the ideal way to make sure the light turns on and off at the same time every day. You just need to make sure that the room is lit either by daylight or room lights for half an hour before the tank lights turn on and half an hour after the tank lights turn off to create a sort of dawn and dusk.

With Leds you can use a ramp timer such as this... http://forums.thinkfish.co.uk/tank-equipment-reviews/nicrew-single-channel-led-lighting-timer/

  I’ll take a look !

(Quote box fixed by Moderator Matt  )

I have that ramp timer but it is the Pro version to simulate dawn and dusk but also control light intensitys.The best £15 I've ever spent.

This Ramp Timer Pro version is a fantastic piece of kit. I can setup a 30 minute sunrise and have 6 different light intensities. Below is the setup I have for my aquarium for 24 hours. This explains and helps anybody. Percentage is the light intensity from that time.

F3 for 30 minute sunrise

L1 8:00 33%
L2 10:00 66%
L3 13:00 100%
L4 20:00 66%
L5 21:00 33%
L6 22:00 0%

Enough brightness in the morning to see and gives fish more calm for awhile. So it's not at full brightness until the afternoon when I properly view my tank.

Not having it at full intensity in the morning stops algae.