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Aquarium Science: Freshwater, Brackish, & Saltwater Environment

22/01/2018 0 Comment(s) Freshwater,Saltwater,Pond & Garden,

Author: Leslie Tsen (B.S.) Ecology, Evolution & Natural Resources. Wildlife Conservation

Water chemistry can be a rather daunting topic even for a well-trained hydrologist. It encompasses diverse parameters ranging from some of the most commonly heard terms such as water pH and temperature to complex ideas which include osmotic pressure and specific conductance. It is because of these parameter distinctions that different waterbody types (eg: freshwater, brackish and saltwater) exist and can be differentiated out from one another. In the fish-keeping hobby, we will only be concerning with a tiny fraction of these topics as there are way too many to describe. At the very least, we will be limiting ourselves to the basic sciences of a few relevant parameters in order to make you understand why housing your fishes and animals in the wrong environment can put their life in great danger.


Before we dive too deep into the exact water chemistry of specific water bodies, we will run you through some of the key parameter terms and concepts so that you will have a better grasp in understanding later sections.
















Freshwater is a type of water body with less than 0.05% salinity, or <0.5 ppt NaCl (S.G. < 1.005). Most if not all freshwater organisms (such as pond snails, river fishes, aquatic plants) flourish in this type of water body. Rivers, ponds, ground water, lakes, streams, and icebergs are some example of places where freshwater can be naturally found. Placing saltwater fish into a low salinity environment (freshwater) can prove to be a fatal mistake, as the low ion concentration water will be forced into the salt ions-riched fish cells, causing excessive swelling known as overhydration. If the condition persists for long, it can be fatal. However, a temporary freshwater bath (known as the freshwater dip) is a popular method used by aquarists to remove ick and other external parasites.


Salinity  : <500ppm / <0.05% / <0.5ppt NaCl

Density  : 1.000-1.005g/L

pH          : Blackwater – 3.5-6.5 | Stream – 6.5-7.5 | African lake / soda lake – 7.4-9.0



Brackish Water


Brackish water is a bridge linking between both freshwater and saltwater regions. It ranges between 0.05%-2.8% in salinity, or 0.5-28 ppt NaCl (S.G. 1.005-1.020). The salt content of brackish water is slightly higher than freshwater but not high enough to be considered as saltwater. Estuaries, mangrove swamps, and brackish lakes are some examples of naturally occurring brackish environments. Three major types of brackish fish exist and some live better under certain salinity during specific life stage. For instance, salmons, sturgeons and striped basses are anadromous fish that are born in freshwater, spent their time growing in the sea, and then finally returning to freshwater to spawn. Catadromous fish such as Anguilla sp. (including American eel and few other species) are born in saltwater, spent their time growing in freshwater systems such as lakes and rivers before finally migrating back to the ocean to spawn. Amphidromous fish such as torrentfish and river goby are born in brackish water/estuaries, drifted into the open ocean as larvae, swam back to freshwater to develop into adulthood before entering brackish water to spawn.


It is very important to note that brackish fish will not do well in a strict freshwater or saltwater environment as their physiology is uniquely adapted to the brackish environment from which they are naturally found. While some species are capable of surviving in either condition, they will not thrive and most definitely will not reproduce due to a reduced quality of life. Therefore, we recommend fellow fish keepers to do a thorough background research before purchasing any brackish fish. If you are unable to fulfil a species’ thriving requirement, do not blindly purchase a fish only to have it suffer and wasting away in your aquarium.  


Salinity : 500-28000ppm / 0.05-2.8% / 0.5-28ppt NaCl

Density : 1.005-1.020g/L (ideally 1.015g/L)

pH         : 7.0-8.5





Sea water is saltwater of about 3.5% in salinity, or 35 ppt NaCl (S.G. 1.025). Most if not almost all marine fish live in this environment. The most recommended salinity for a marine tank is of course one that is closest to that of seawater - 3.5% salinity with a specific gravity of 1.025. Corals however, seem to prefer a slightly saltier environment (S.G. 1.026-1.027). Lagoons, open oceans, and salt lakes are some example of places where you can expect to find naturally occurring saltwater in nature. Freshwater fish that are being placed in saltwater for too long will lose water to the surrounding and eventually die from dehydration. Regardless, a temporary saltwater bath (known as the saltwater dip) using low salinity water for a short period of time can help freshwater fish to generate protective slime coat and fight off protozoa parasite.


Salinity : 29000-50000ppm (33000-37000ppm) / 2.9-5.0% (3.3-3.7%) / 29-50ppt (33-37ppt) NaCl

Density : 1.021-1.037g/L (1.021-1.027g/L)

pH         : Ocean – 8.0-8.4 (8.1-8.3)

*Values in red are ideal figure ranges that support the most lives.


Very Salty Water (Brine): 

A brine is a type of saltwater with more than 5% salinity, or >50 ppt NaCl (S.G. >1.037). Very few complex organisms are able to live in this type of environment since the environmental condition is so harsh. However, a small handful of salt-tolerant and salt-loving species have adapted physiologically to thrive in these environments. Some examples of natural brine are the Mono Lake in California, Great Salt Lake in Utah, Baltic Sea in Northern Europe, and the Dead Sea to the East of Isreal. Due to the unique mineral composition of dead sea salts which differ significantly from that of ordinary oceanic salts, dead sea salts have been shown to exhibit various therapeutic properties against many skin conditions.


Fun Fact: The dead sea is a hypersaline saltwater lake that comes with an astonishing 34% salinity. It weighs 1.24kg/l and is so dense that humans can even float on the water surface with ease! 


*Values in red are ideal figure ranges that support the most lives. Note: World's Ocean has a salinity range from 33-37ppt NaCl.

Types of Water (24.4°C)

Salinity (%)

NaCl (ppt)

Specific Gravity






Freshwater species

Brackish Water




Brackish species


Lower Limit


(33 - 35)

(1.023 - 1.025)

Marine species

[Indian | Pacific | Arctic - Fish & Corals]

Upper Limit


(35 - 37)

(1.025 - 1.027)

Marine species

[North / South Atlantic - Fish & Corals]

High Salinity




Slight Halophiles

(salt-tolerant organisms, eg: bacteria)





Moderate-Extreme Halophiles

(salt-loving organisms such as bacteria, archae & brine shrimps)

Table 1: Water bodies and their corresponding general water chemistry. Written by Leslie Tsen.

**Sources: Images are accredited to their original creators; otherwise, fair use for educational purposes. All icons credit goes to their original creators from FreePik and Flaticon

Disclaimer: This article contains materials referred from published scientific journals and is strictly meant for educational purposes only. It must NOT be substituted for any forms of medical care, treatment, and consultation from veterinarians, aquatic experts or other licensed professionals. NO compensation shall be reimbursed by Harvest Fish & Pet for the direct or indirect loss, damage, injury, or death of both living (user included) and non-living beings caused by the application of information from this article either in full or in part.  

Tags: Science