What Should The Salinity Be In My Marine Aquarium? Creating a Perfect Environment

Welcome to Simple Reefs. We are a brand new website designed to take the stress and expense out of starting your very own marine aquarium. We’ve been talking about Salinity a lot, lately. That got me to thinking, perhaps I should put together an article answering a really common question – What Should The Salinity Be In My Marine Aquarium?

Now, this answer isn’t quite as simple as you may initially think. After all, there isn’t, strictly speaking, a one size fits all number when it comes to salinity. Certain fish prefer a number that is a little lower while more delicate coral and clams like it to be a little higher.

Instead, what we aim to do is to hit a happy medium that suits all the residents of our tank. This happy medium typically corresponds with the average salinity of the sea which is 35 ppt. The truth is, the sea varies in different parts of the world so even this isn’t perfect science. Before we answer the question of what should the salinity be in your marine aquarium, let’s take a look at the importance of getting our salinity right.

A Handy Salt Mixing Calculator

Before we start, I wanted to introduce you to the Simple Reefs’ handy salt mixing calculator. This should make the whole process of working out how much salt you need to add to your freshwater a hell of a lot easier.

The Importance of Correct Salinity

Salinity is an incredibly important metric when it comes to marine aquariums. After all, the creatures inside your tank have evolved and adapted to live in a salt rich environment. Salinity that is too high or too low can cause massive problems in your aquarium. We talk all about it in this article right here.

To quickly summarise, the salt in your marine aquarium isn’t just your bog standard table salt that you would season your dinner with. It is a complex mix of all the foundational elements that allow life in your aquarium to thrive. This includes magnesium, calcium, potassium, bicarbonates, and trace elements, among other things.

The main reason why salinity levels are so important in your marine aquarium is because the life inside of it has adapted to living in a salt rich environment. These creatures are constantly working to regulate the salinity of their bodies with that of the surrounding water. Having the correct salinity in your water is crucial for something known as osmotic balance.

A Delicate Balance

If the salinity in your water is too low, water will rush into the creature’s cells which will cause them to swell (osmotic shock). If it is too high, the water will be drawn out of the creature’s cells causing them to dehydrate.

Both of these situations will cause immense stress to your aquarium’s residents as they attempt to rebalance the salt inside of their bodies. This can lead to sickness and premature death. Coral, in particular, are very sensitive to fluctuations in salinity. Salinity can also have an impact on things like Alkalinity and levels of elements like Calcium and Magnesium.

This naturally raises the question “what should the salinity be in a marine aquarium?”. Well, there are a couple of different answers to that question and it is a tiny bit more flexible than what you may think. First of all, however, let’s talk about how we actually measure salinity in a marine aquarium.

There are two commonly accepted standards when it comes to measuring salinity. They are SG (Specific Gravity) and PPT (Parts Per Thousand). Well, there is also a third known as PSU (Practical Salinity Units) that works as a numerical equivalent to PPT but it isn’t referenced a great deal in the commercial marine aquarium hobby.

The two units you will see most often are SG and PPT. They both work a bit differently but they both arrive at the same result. One just happens to be a little easier to work with than the other. We talk all about both of them in this article right here so if you want a more in depth explanation, check it out.

What Should My Marine Aquarium’s Salinity Be?

This question depends on what type of aquarium you have. Different setups have different needs which is where things can get a little confusing. This is particularly true if you started with one type of marine aquarium but then decide to move onto something else.

It also gets a little confusing when we discuss fish-only but don’t talk about the inclusion of clean-up crew like shrimp, crabs, and snails. I almost never see this defined by anyone who talks about the subject. A fact which feels a little strange as I don’t think the definition of Fish-Only implies that enough. Particularly when most places tell you to start your aquarium with a clean-up crew, regardless.

Suffice to say, if you have a clean-up crew, you will want to shoot for salinity closer to Reef Tank requirements as these creatures are less tolerant of lower salinity levels. When we say fish only, we really mean fish only.

I would suggest, if you are starting with a fish-only aquarium but have even the slightest hint that you might want coral in the future, aim for slightly higher salinity to start with to make the transition easier in the future. It will be easier and less stressful on your fish to swing a small amount rather than a high amount.

Fish Only Aquarium Salinity

Salinity can be maintained slightly lower in a fish-only aquarium. Most hobbyists typically opt for between 28-33 ppt (SG 1.020-1.024).

Lower salinity levels actually have a few benefits that you might not expect. Naturally, we can look at cost as the first benefit, right? If we are using water with a lower salinity it is going to cost us less in actual salt.

If you are running a fish-only aquarium, you can opt to purchase salt that is more affordable, as well. Most marine aquarium salt designed for reefs will be fortified with additional elements. Basic “Sea” salt is usually about 10-20% cheaper.

Believe it or not, lower salinity can reduce stress on fish. It can also work to reduce the risk of certain parasites spreading out of control. Some people actually use hypo-salinity (very low salinity) while quarantining fish as the fish will usually tolerate the low salinity levels while the parasites won’t. Whether this is effective, or not, is a matter for debate.

Certain fish may not appreciate very low salinity while others are quite tolerant of big drops. Some, like Clown Fish, are capable of living in surprisingly low levels of salinity, though this is obviously not recommended.

In a fish-only marine aquarium, you probably want to aim for a happy medium of about 1.021-1.022 SG (29PPT – 30PPT). This will afford you some big money savings and allow your fish to thrive. It will also allow you a window to easily adjust your salinity over time if you want to start adding coral.

Please Note: This low salinity is for Fish-Only. That means no invertebrates or molluscs. If you want to add snails, shrimp, crabs, etc., you need to jump up to reef aquarium numbers as they are intolerant of low salinity.

Basic Reef Aquarium Salinity

For tanks with fairly basic coral and invertebrates, salinity should be kept at 33-35 ppt (SG 1.024-1.026) to mimic natural seawater conditions. If we are talking about soft coral, here, and a very basic reef aquarium, you can aim for the lower number to cut costs just a tad.

The main thing we want to consider here is stability. That is the key to a healthy reef, even with basic coral like softies. You don’t want fluctuations as the environment on the reef really wouldn’t experience much in the way of shifting salinity.

Some corals that are present in shallower waters may be a little more tolerant of shifts due to the nature of tides. The best way to consider your aquarium, however, is that it exists at the bottom of the ocean and changes are minimal.

There is some personal preference here depending on how your tank is set up. I think a safe salinity to shoot for and one that a lot of people cite as the standard is 1.025SG (34PPT). We might want to move that up a hair for more advanced tanks, though.

Advanced Reef Aquarium Salinity

While it is absolutely fine to opt for 1.025SG (34PPT) with an advanced reef. It might be better to shoot just a little higher and go for 35 PPT (1.026SG) as this is the salinity of the sea water that many stony coral and clams have adapted to.

I find that this salinity level allows for a slight swing in the negative without slipping outside of what would be recommended for a reef aquarium, as well. This might be just enough to give you a window to fix the problem.

As always, stability is key. In fact, in a more advanced reef aquarium with more delicate coral, stability is of paramount importance. Stony coral and clams are very sensitive to fluctuations of salinity. I would definitely suggest upping your salinity checking to every other day or even daily depending on preference.

Fluctuations and Swings in Salinity

There are a number of different factors that are going to contribute to swings in your salinity. Some of them are everyday issues that are to be expected and should be dealt with (evaporation for example). Others are less predictable and can come as a result of user error or equipment failure.

We are already over 1500 words here. I really want to elaborate on both the causes and solutions to these problems in a thorough way because the life of your tank’s residents depends on it. With this in mind, I will be putting up another article very shortly on the causes of salinity swings and how to deal with them.

In Short

The salinity of your water is incredibly important to the residents of your tank. Try to aim for a salinity of 1.020 – 1.024SG (29 – 34PPT) for fish only aquariums and 1.025-1.026SG (35-36PPT) for reef tanks. Maintaining a consistent salinity is very important to your reef tank’s inhabitants. Thanks for reading and spending your time with Simple Reefs.