I recently had my water tested at a local petshop, and was told that although everything else tested great, the salinity of my tank was too high. I asked what the reading was, and was told that it was “way up” at around 35 parts per thousand (or roughly 1.026 for specific gravity). I tried to explain that was the salinity at which I normally keep my tanks, and was told that "at that salinity the available oxygen in your tank will be so low that it will likely kill your fish." I must admit that I was shocked to discover that the natural salinity of seawater was “too high” and could be potentially lethal to my tank inhabitants!
Sarcasm aside, I often hear arguments that either the salinity or temperature of an aquarium should be kept lower than natural for a variety of reasons. Some of these reasons sound very plausible, but I have yet to see any research showing that coral reef animals do better under these conditions than those at which they are found in the wild.
Let me first back up a step and provide the background for why this information even matters. Coral reefs occur in only a small proportion of the oceans, and although there are many explanations for why that may be the case, the temperature and salinity are relatively similar in all those locations. For zoos and professional aquaria, the standard method of operation is to duplicate, as closely as possible, the natural conditions in which the animals are found living in the wild. That seems like a reasonable idea, but the vast majority of people keeping salt water tanks don’t actually follow this simple common-sense rule. When deciding how salty or how warm to keep an aquarium, it seems like a reasonable place to start would be to find out what the salinity and temperature are in nature where the animals we keep live wild, right? So what are the average temperature and salinity of the waters around coral reefs in the wild? On average, the ocean around tropical reefs ranges from 35-38‰ although much of the Red Sea and Persian Gulf exceed 40‰. There are more animals imported from the Red Sea now than ever before, but the vast majority of livestock in North American petshops still comes from the IndoPacific or Caribbean, so a salinity of about 35-36‰ (or roughly 1.026 - 1.027) should be considered the natural value for these animals. What about the temperature? Taking values compiled by Ron Shimek, the annual temperature (averaged across both summer and winter water temperatures) of Caribbean reefs ranged from a low of 79.7ΕF in the Florida Keys to roughly 83.7ΕF with a maximum daily temperature of 91.4ΕF for Belize. For the IndoPacific, the values ranged from a low of 80.7ΕF across the Great Barrier Reef to 83.3ΕF at Enewetok where the hottest summer days result in water temperatures as high as 92.5ΕF! Overall, the temperatures in the summer are a few degrees warmer than these averages and the temperatures in the winter are a few degrees cooler, but something around 82ΕF is close to the annual average temperature for coral reefs. Well, if you glance at the back of any bag of artificial salt, you’ll find the a recommendation to maintain the specific gravity at somewhere around 1.020 (or roughly 27-28‰) to 1.023 (or roughly 31-32‰) at 75ΕF (~23-24ΕC). Read through a few classic aquarium texts and you’ll find basically the same recommendations, too – so why is that? Those recommendations are certainly not the same as the natural conditions, and in fact, I doubt that there is not a single coral reef in the world, which is found at those conditions.
Personally, I think this standard recommendation has to do with tradition – there are a lot of things in this hobby that are carried forward because of tradition rather than science. There are plenty of reasons given why the tank temperature and salinity should be kept within this range, and depending on the book or magazine you read these range from “that’s how it has always been done,” to the scientific-sounding reason given above – there isn’t enough oxygen in the water to support our animals if the salinity and temperature are kept as high as found in nature. Rather than go through all the reasons I have heard and why they don’t make any sense to me, I thought I’d just go through this one example – “at that salinity the available oxygen in your tank will be so low that it will likely kill your fish.”
The amount of oxygen, which can dissolve in salt water depends on the exact salinity and temperature of your aquarium -- the table below gives you the values of oxygen saturation (mg/l) across a wide range of temperatures and salinities. The basic answer is that saturation for an average marine aquarium is roughly 6.5 mg/l, but that this value means relatively little to the overall health of your animals (which I will explain in detail below).
Salinity (‰ – parts per thousand)
Temperature 0‰ 10‰ 20‰ 30‰ 40‰
10°C (~50°F) 13.0 12.2 11.4 10.6 9.8
15°C (~59°F) 10.3 9.7 9.2 8.6 8.1
20°C (~68°F) 9.4 8.8 8.4 7.9 7.4
25°C (~77°F) 8.5 8.0 7.6 7.2 6.7
30°C (~86°F) 7.8 7.4 7.0 6.6 6.2
At this point it makes sense to ask whether out tanks need to be at saturation, and what normal levels of oxygen are in the ocean in areas around coral reefs. It makes sense that surf zones are probably close to saturated with oxygen, but what about the areas around coral reefs – how much oxygen is in the water there? Well, natural seawater in areas around reefs are generally between 95-110% saturated with oxygen, and the majority of areas measured are at 100% saturation, so it seems reasonable that level of oxygen which can dissolve in seawater may be important to an aquarium. Obviously oxygen is important to our animals, and if water around natural reefs are usually saturated with oxygen, then perhaps that argument has some merit?
Well, I don’t think so. Measurements around one Caribbean island showed that night time oxygen levels range from about 5.0-6.5 mg oxygen/l, and daytime levels rise to around 7.5-9.0 mg oxygen/l (saturation was roughly 6.25mg/l) depending on the reef measured. Daytime oxygen concentration rise to above saturation because photosynthesis produces oxygen at a higher rate than it can leave the water by diffusion, and the areas of lower oxygen concentration are actually those with the most gas exchange, because oxygen diffuses out of the water more rapidly and is closer to the saturation level of about 6.25mg/l than areas with limited gas exchange. The opposite is true at night, when respiration uses up available oxygen and the areas with the lowest gas exchange depress the oxygen concentration farthest below saturation. Walter Adey reports similar levels of oxygen in his ATS-based tanks at the Smithsonian, but is also quick to point out that it is not the oxygen concentration itself that is important but rather the exchange rate of oxygen that is important to the health of the animals.
Let's say you start with an average well-stocked 50G reef aquarium. The respiration of the animals in the aquarium would likely be on the order of 3g of oxygen per hour. Even if you could supersaturate the seawater (lets say it's at 9mg oxygen/l -- the highest recorded on the natural reefs I mentioned), that still only gives you about an hour before the animals suffocate if gas exchange is limited. Of course, protein skimming, turbulent water flow (e.g., "dueling" powerheads, and especially surge devices), and photosynthesis will alter that rate of exchange, and with the normal exchange rates of roughly 4-6g of oxygen per square meter of surface area per hour, the respiratory needs of your animals should be easily met. While it is true that both salinity and temperature will affect the particular value of the oxygen saturation coefficient in seawater – e.g., if you're being good and maintaining your tank at a natural salinity of ~35-36‰, then the oxygen saturation at 75ΕF (~24ΕC) is roughly 6.8mg/l, while at 82ΕF (~28ΕC) it drops to only 6.6mg/l) – this is pretty much a non-factor. Given that natural reefs can fluctuate in oxygen concentration from around five to around nine on a daily basis, it seems a little silly to me that anyone should worry about a difference of 0.2 mg/l in the middle of that range. If this small difference in oxygen saturation point makes any difference to the inhabitants of your tank, you have other problems, and must be seriously under-circulating or overloading your tank! As long as your dissolved oxygen level remains above about 5 mg/l then there is no reason for concern with the oxygen level of your tank, and even if there were, it is the rate of gas exchange in the aquarium rather than the total amount of oxygen that is able to dissolve in seawater at saturation that is of concern to the health of our animals...