Brandon has always loved aquariums. Right now he's got a heavily planted Dutch style aquarium with different livebearers and some corydoras
Fish express how they feel through behavior and changes in appearance. In this article, I'm going to be covering a peculiar change in guppies—the blackening of their eyes. There are quite a few scenarios where this happens, luckily most of them do not require any action from your part.
Causes for the Blackening of Guppy Eyes
I must point out that this is an active research question and we now know of quite a few possible reasons for the eyes of a guppy to suddenly turn black. Since the area of research is relatively new, it would not be surprising if the reason for your guppy to have blackened eyes is not enlisted here.
1. Dominance and Willingness to Be Aggressive
In the wild guppies can get very protective of food, especially when they are in isolated pockets of river water during the dry season where food can get scarce. Robert J. P. Heathcote et al. published a research paper titled "Dynamic eye colour as an honest signal of aggression" that looks into dominance and aggression as a cause for black eye rings.
The researchers needed to be able to replicate the eye coloration in a test environment. They made realistic-looking robotic guppies with and without the black ring around their eyes. They had to make sure that the robotic guppies looked real to the actual live guppies and not just to the human eye.
These robotic guppies were then placed over food and made to appear as though they were interested in the food or were guarding the food. The researchers observed that the live guppies were more likely to try to get a bite off the food if the robotic guppy had a silver ring than when they had a black ring around their eyes.
Also, if the robotic guppy with a black ring was smaller than the live guppies, the smaller robotic guppy was statistically attacked more often than the guppies with a silver ring.
The conclusion of this experiment was that guppies nearly instantaneously turn their eyes black if they are protecting food, but they only do this if they know they can win a fight. If they fake strength (when they are smaller), making their eyes black only draws unwanted attention to the food and they are more likely to be attacked instead of keeping the other guppies at bay.
I tested this out: I have noticed this with the female guppy in the picture above. She is the largest that I've got, being pregnant makes her seem even larger. After a day of fasting which I tend to do once every fortnight, I fed just one sinking tablet (insect and algae-based) and the female guppy and the other fish darted towards it,. The female, however, darkened her eyes immediately after the tablet settled on the gravel.
She fought off the corydoras and the other guppies and endlers. The platy and molly however were not attacked (they were similar in size to her). The mollies fought off the platys, other guppies, and corydoras too, but did not attack the female guppy. It was interesting to notice the mollies and the female guppy attack the food together keeping the others at bay. It seems like the black eyes serve as a danger signal across species, too. I only say this because the corydoras are larger than the guppy and the mollies attack them but not the female guppy.
I almost immediately added more food, another tablet, and some flakes. But the guppy continued to have darkened eyes until she was done feeding. I have repeated this three times so far. The other times where I first added a lot of flake food spread on the surface of the water she did not turn her eyes black, probably because there was nothing she could protect. She devoured the flakes instantly, whereas the tablet was large and she could eat off of it.
Most aquarists associate the sudden darkening of the eyes of a guppy to stress. If you read through forum posts on the topic you will notice people talking about water parameters covering everything from the pH and temperature to the nitrogen cycle and its associated ammonia and nitrite levels.
I disagree with these posts, because in all the posts that I've read most of the guppies continue to have a silver ring while just a few have black rings. If it were truly a water parameter most would have a black ring. Secondly, the eyes would continue to remain black until the water parameters changed, but people comment that the blackening is intermittent.
What I have observed: The picture you've seen above is one where my female guppy has her eyes turned black due to stress or a sign of aggression. This happened because three male fish were chasing her around for quite a while. It was possibly a sign of stress or she was just pointing out that she's ready to fight them if they get closer. I am more inclined to lean towards stress because she did not fight them, but kept running around trying to get away from them.
I added a few more female guppies to my tank after noticing the elevated stress levels. Things are much better now. I thought it would be wise to add them in a 1:1 ratio, I now have the male to female ratio as 1:1.5. This is the only case where I'd say you can actually intervene to help lower stress levels (with the black eye symptom) in your fish.
I have also noticed one of my male guppies gets blackened eyes at times. This is whenever he is being bullied by my male cobra guppy (seen in the first picture above). I must point out that this guppy was weaker because he had camallanus worms and was already bullied and had a damaged tail fin when I bought him at my local fish store. I treated the entire tank for the worms and fin rot too. This is another instance where I observed black eyes potentially due to stress.
3. Natural Coloration
Some of the Moscow guppy varieties tend to have black rings around their eyes instead of silver as seen in the other varieties. This is their appearance in their natural, non-stressed environment and is therefore not a cause for worry.
There are instances where people end up with some of their fry having a constant black ring. This is not a cause for worry as there are going to be instances where a guppy is going to have a higher proportion of melanophores which makes their eyes seem black.
4. To Trick Predators
Relatively new research by the University of Exeter has shown that guppies trick predators by turning their eyes black.
We noticed that guppies would approach a cichlid at an angle, quickly darkening their eyes to jet-black, and then waiting to see if it would attack.
— Dr. Robert Heathcote
If the predator is hungry and willing to attack the guppy, it will be attracted to the head due to the dark coloration and when the predator strikes the head the guppy can dodge relatively easily when compared to a strike at the mid of its body.
Since this article only covers a list of possible reasons behind the sudden black coloration, I do not want to describe this experiment in detail as it has already been done by others. You can read the research paper here or check out a summary of the paper here. The summary also includes a video.
This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.