Within the aquarium hobby there are not many cases of husbandry that are used to improve a fishes appearance beyond maybe a red enhancing diet or just general good care.

It’s a little difficult from any of the grooming you can do with other animals, we can provide enrichment and exercise but again it’s not quite the same. Although in the goldfish hobby there are definitely quite a few practices aimed at improving a fishes condition from certain shaped containers to lighting.

The topic for this article is about lighting, more specifically whatever maybe called natural lighting as do we even know what we are referring to by that? The use of lighting is not just used within goldfish but also Asian arowana, discus maybe to a lesser extent.

Regarding discus just to get it out of the way, their colouration is dictated easily by the brightness of the setup so not strictly lighting but that is a massive contributing factor. In a dark setup the fishes will change to a darker patterning, if possible they will produce those solid bars of pigmentation from dorsal to ventral. In wilds such as S. tarzoo they go much darker as in the photo below:

Yet in a much lighter setup you can see a lot more of the fishes beautiful colours.

This is not just strict to wild discus who likely use their chameleon abilities to communicate and blend in with their environment, it is found in domestics too. There is one issue there, there are many varieties, the most popular being the pigeon bloods who cannot colour change as quickly. These varieties instead develop what is known as peppering, black spots on the body and as while they can disappear with brighter lighting it takes a lot more effort and time. Maybe this is the answer for anyone wondering why discus aquariums look the way they do?

This is a difficult topic to research scientifically though, mainly because it effects the hobbyist and really will lack any funding to support research. There is a little though in some species and through the review of Leclercq et al. (2010) we know that UV light particularly UVB contributes to colour changes in many fishes followed by the background unlike discus. UVB is additionally the spectrum of light that causes tanning in humans, similar to goldfishes where there is a predisposition to black pigmentation either get a higher intensity of black pigmentation or it grows.

The colour changes could be a result of UV protection given there are levels that penetrate depending on the water turbidity and location (Mueller & Neuhauss, 2014). While maybe there is other causes to this change in colouration? Either way it is utilised quite a bit in the hobby.

So why the title? I could get away with it with just mentioning goldfish but tanning of fishes is well known with the dragon fish, the Asian arowana, Scleropages formosus to produce intense deeper colouration.

So how is it done? In some countries or in the case of goldfish having ponds outside will do the job, natural sunlight not passing through glass but obviously this is not possible for many of us. UVB being the likely main aspect of lighting that we need to aim for lighting needs to go out of the realm of fishkeeping and into herpetoculture. In the care of reptiles UVB lights are quite common and these should do the job, what I cannot recommend is how much and how little to use and I feel personal experimentation would be important. Bare in mind reptile UVB lighting requires replacing every 6-9 months as it only emits UVB for so long.

I have experimented with a variety of lights and with goldfish found nothing compared, confirming UVB’s importance in my personal opinion. I have found fluorescents although producing no UVB much much better then LED’s apart from in the case of dealing with fishes like discus that want brightness rather then certain aspects of the spectrum.

Considerations are certainly needed, it would not be ideal for the lighting to be on constantly so not to mess with the fishes circadian rhythm. Another aspect is there is too much UVB and fishes might feel they do not want to be exposed to bright light constantly, I would certainly give them refuge from the light in terms of some cover that should they want to they can retreat to.

For understanding UVB but sadly in the context of reptiles I recommend this video: https://www.youtube.com/watch?v=L83RApPPri8

References:

Leclercq, E., Taylor, J. F., & Migaud, H. (2010). Morphological skin colour changes in teleosts. Fish and Fisheries, 11(2), 159-193.

Mueller, K. P., & Neuhauss, S. C. (2014). Sunscreen for fish: co-option of UV light protection for camouflage. PLoS One, 9(1), e87372.