Goldfish with tropicals - right or wrong?

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Goldfish with tropicals - right or wrong?

Continuing his theme of controversial topics, Nathan Hill decides to approach a closet taboo of the trade - the goldfish in a tropical tank.


Let me make one thing clear from the offset - the answer to the debate about keeping goldfish in tropical tanks is not an empirical one, and I cannot give a bona-fide solution to it being right or wrong on scientific reasons alone. However, as shall become clear, I am opposed to it for reasons that we shall see.

On the back of this article, I fully expect that there may be responses from those keen to highlight the fact that their goldfish that resides within their community tank is quite happy, and that my conclusions are wrong. But irrespective of this, my argument is not one of whether the fish can survive or thrive in such an environment or not, but only whether it is something we should be keen to do.

Tropical paradise

I want to start by clarifying a few things that I feel are relevant to this blog. Firstly, let me draw out a distinction to what constitutes tropical and temperate - and for anyone who wants to really get my blood steaming - 'coldwater.'

Strictly speaking, when we make reference to the words tropical or temperate, what we are actually talking about is a reference to a specific region, and not in particular a temperature. So, when I use the phrase 'tropical' then what I actually mean is in relation to anything that originated in the tropics - between the tropic of Cancer and the tropic of Capricorn.

When I use the phrase 'temperate' I refer to anything originating from the temperate zones between either of the two tropics, and the arctic or Antarctic circles.

When I refer to 'coldwater,' which I won't, I'm talking about nothing of relevance to the blog. Personally, I think that one of the worst things that happened to the industry was the recognition and common usage of coldwater to indicate something that is actually temperate. The word has a use, for sure, but certainly not in the day to day affairs of an aquatic store. If I use it again in this thread, you all have permission to punch me in the kidneys.

Now, a tropical region, by virtue of it being in the tropics and nearer to the equator, is generally accepted to be warmer. However, that's not to say that everywhere in the tropics is warm. There are some rather cold mountains there, to contrast the deserts and humid rainforests, and hypothermia is not unknown just because you're in a tropical zone.

A temperate region, in being further from the equator, is generally considered to be cooler, but more importantly it has a much wider range of temperatures than we find in the tropics. Moreover, given the frequency with which temperate regions fluctuate in temperature - they are after all much more heavily affected by seasonal Earth wobble which only has minimal effect on the tropics - it is fair to assume that anything living in them will be subject to a greater set of extremes than their tropical cousins.

Where the hell are you going with this, Nathan?

Okay, so now I've established the differences between tropical and temperate regions, hopefully we can all now see that to merely come from one region or the other is not to come from a set temperature. There are too many variances within each zone for anyone to simply lay down a law that states that if something is tropical it needs water of 25°C, and if it's temperate then it needs 18°C, for example. It doesn't work that way.

Now, our humble goldfish is technically a temperate fish. Originally it inhabited a region in which the temperature of the water was not a constant giddy high of 25°C, and as we all will know, it exists in a wide range of temperatures. They are now found in unheated aquaria, heated aquaria, outdoor ponds and rivers, and even outdoor water features, where temperature change can be drastic and fast.

Now, on one strand, I could kill the argument here. I could set up a tropical biotope in which the water temperature may only be some 20°C - say a mountainous stream tank from somewhere in Asia. This would tick the definition of a tropical tank in the strict sense, and yet it is at what is often considered the ideal temperature for goldfish, with 20 - 22°C usually considered to be their peak.

But I'm not going to do that, because I'm enjoying myself too much. Instead, I'm going to press on with the universally accepted (and incorrect) view of what it is to be tropical, and that is the assumption that anything from the tropics lives at a steady 25°C, unless you happen to be a Discus in which case you only inhabit waters closer to 30°C.

Now assuming that most people have a 'tropical' tank that runs at this temperature, we can now form an argument as to whether goldfish should or shouldn't be kept in it.

What's the problem, hot stuff?

Goldfish are poikilothermic. This simply means that they can change the temperature of their body by changing environment, but moreover they can adapt to temperature changes. This contrasts with a homeothermic animal that has no ability to function at anything outside of a very tight band of temperatures. One might argue that a fish from a coral reef is homeothermic, and most corals certainly fall into this category.

Now, just because a goldfish can adapt to a wide range of temperatures, it doesn't mean that it does so without harm, or physiological changes.

The biggest change to a goldfish at varying temperatures is its metabolic rate, which we tend to look at in both digestive and locomotive demands on the fish.

To put this simply, as a goldfish is subject to increasing temperatures, its body requires more food, more oxygen, and crucially the fish produces more waste.

When talking of metabolic differences, I'm not referring to paltry changes. I'm talking about big changes. So, for example, one typical study shows that the difference between metabolic demands of goldfish at 25°C are over 140% more than a goldfish at 15°C. And that means a lot more waste comes out of a warm goldfish than a cooler one.

Not only that, but demand for food increases in tandem with this. One of the biggest issues with goldfish in a 'tropical' tank is undernourishment. Food goes in, more food goes in, but still the goldfish can't get enough to maintain its weight, and so emaciation and ill health follows.

Obviously, one could crack the feeding right up and keep the goldfish happy this way, but what is the cost of doing so? My problem here would be one of the sheer amount of TAN (total ammonia nitrogen) being produced. In real terms, big meals = loads of ammonia, nitrite and nitrate = loads of waterchanges = messy tank if keeper too bone idle to change frequently = unhappy fish = unhappy Nathan = fight in public.

However, there's little evidence to suggest any direct physical issue in goldfish at 25°C. Aside from the waste production, there's little to suggest that the fish digests any differently, or produces any toxins that wouldn't be normally expected. Studies upon the liver of goldfish at 25°C find no evidence of hyperthermic problems - although that changes a tad when things creep up to over 30°C. Here we start to see the early signs for the destruction of enzymes, and that's not good for an organism that needs its enzymes to stay alive.

A bigger issue is seen physiologically with sudden, fast changes of temperature over a chronic high temperature, and it appears that more damage is wreaked in moving a goldfish more than 3°C in a 24 hour period than keeping it at a constant of 25°C.

Interestingly, goldfish actually change their muscle fibres according to the temperature they are kept at, with those kept in cooler water developing much more pink and red muscle fibres than those that do not.

This corresponds to an increase in the (brace yourself) mitochondrial oxidative capacities of goldfish cells, which may not be a good thing. Increased mitochondria in these cases leads to an increase of free radicals in the body. If you've ever been suckered into the 'detox' adverts that tell us that antioxidants will make us live for two hundred years, and remove all the free radicals from our body, then you'll know roughly what I'm on about here.

But in a chestnut, low temperature could be leading to an increase in cellular damage through oxidation.

However, there's a totally separate argument involving mice. The parameters are so flimsy when connected to fish that I'll mention it only in passing, but it's handy to know for pub quizzes, and mentioning if you want to sound like you know what you're talking about.

In trials on mice, it was found that lowering their internal body temperature increased longevity. It didn't move far, only 0.2 to 0.5°C lower than usual, but the result was that female mice were living up to 20% longer than their warmer counterparts.

Does this provide any evidence that a cooler goldfish can live longer? Well, no. Not in the slightest, but at least you'll be ready to challenge this claim if it ever comes up.

The Nathan argument

So far, all I've established is that biologically there seems to be no pressing issue on the quality of life of a goldfish in a tank of 25°C, aside the production of so much waste, and the demand on the fish, metabolically speaking. However, it's not enough for me to rally the people and attack my local aquatic store with pitchforks and fire.

My argument is simply why would you want to keep a goldfish in a tropical tank? I mean, really, just why?

Call me a pretentious purist, but I'm more interested in getting my act in line more for the biotope, or at the least the nature aquarium in which I have a harmonious selection of fish that don't intimidate each other. To me, the idea of putting something together that has a different set of requirements to something else is abhorrent, but that's merely personal taste.

Don't get me wrong, I'm no goldfish hater. I do resent what we have done to them over the years, but I've nothing against the fish itself. If going for a goldfish, why not try to keep it at its very best - at its optimal parameters.

Even better, why not consider a riverine style tank for goldies alone? Avoid the garish path of coloured gravel and 'no fishing' signs, and try to create something more natural. After all, isn't fishkeeping about putting the fishes' best interests at heart? If not, and if the fish is just a decorative feature to spice up the living room, then I'll probably not get through to you.

So ultimately, that's where I stand with it. You pay your money, you take your choice. Somehow, the idea of goldfish in tropical aquaria still doesn't sit right with me, for reasons that I can't quite place my thin little digits upon, but I'm happy to concede that I can't find an absolute reason why it shouldn't take place.

So taking into account the following exception, admit it; you spent most of that article seeing if I wrote 'coldwater' (excluded from kidney punch) again after I said I wouldn't. I don't think I did, but I'll be on my guard.

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