Brackish & Freshwater Definition

N

nattybrack

AC Members
Jan 13, 2003
96
0
0
NYC
Really basic question but... The way I understand it, brackish water has some salinity, giving it a specific gravity >0. Freshwater does not have any salinity. Cichlids require a specific gravity of roughly 1.004. Cichlids are freshwater though? Also, is the only difference btwn fresh and brackish waters the salinity? Is the only thing that makes water brackish salinity?

Much Appreciation
 
Well, first of all the specific gravity of water is `1.0. The definition of specific gravity compares the density of a liquid or solid to the density of distilled water-- at a temperature of 4o celsius to be exact, the densest water ever gets. Thanks to this atomic quirk, ice floats. And warmer water floats on colder, denser "bottom water" in all the world's oceans.

So the specific gravity of the most alkaline freshwater lakes, like Tanganyika, isn't all that far from the specific gravity (`~1.000+) of the "freshest" fresh water. It was the apparent difference between specific gravities of "0" and of 1.004 that sparked your question, I think.

Some cichlids are specialized to deal with high total dissolved solids (TDS), and some Etroplus cichlids of India and Sri Lanka (the "Chromides') may grow in somewhat brackish estuaries and lagoons.

But Cichlids are not brackish fish.

Only a few fish actually remain "brackish" water critters for long. As they mature, almost all the estuary fishes head towards the ocean. None head upsrtream to fresher waters except eels, which have been sojourning in brackish estuaries while they pass through their last juvenile stage, then head upstream to spawn and die.
 
first of all, I don't exactly know the Salinity definition of freshwater and brackish water, but I'll tell you what I know. I've read somewhere that all fish requires salt in their system, but freshwater fish's need for salt is usually satisfied by tapwater, and a lot of time there is a little bit more salt in your tap water than there is in their natural environmen. There shouldn't be any fresh water supply where the specific gravity is at EXACTLY 1.000000000.... (and so on) unless it's pure H 2 O. By cichlid, I'm sure you meant African cichlids, it is true that lake malawi, lake tanganiyka, and lake victoria has a bit more salt (and also minerals) than most freshwater streams where most tropical fish comes from, but it shouldn't be all that far off from your water supply. As far as the difference between freshwater areas and brackish water areas, besides salt, I only know that there is also higher amount of mineral in brackish water, which usually contribute to higher pH as well. If I'm not correct on any of the information, feel free to correct me.
 
so therefore, even though cichlids live in water with a specific gravity of 1.004 or close thereby, they are not considered brackish because the specific gravity is not much different than freshwater which has 1.000 or close to it. Whereas a brackish fish would have a greater specific gravity.

Thank you for the clarification. Not to beat a dead horse, but at what specific gravity would a fish be considered Brackish? Also, you mentioned TDS, I'm assuming these are the minerals etc... inherant in the water.
 
I would love to see a reference fo any cichlid other than the Asian Etroplus species that live in water of specific gravity 1.004.

The rift lakes of Africa are mineralized, but Malawi and Tanganyika are not that hard. Both have higher salts (by chemical terminology) than many freshwater lakes, but Malawi in particular has almost no NaCL, sodium chloride, or "salt" in aquarium terms.

Some CA waterways are quite hard, but not 'salty' as in brackish until they reach their estuaries.

Brackish should in aquarium terms refer to tidal or estuarine environments where the added minerals are the FW dilutions of SW. Aquarium water with NaCl added may be refered to as "salted", but it is not brackish.
 
Last edited:
Also understand that there are many things which will alter the sp of water besides the addition of salt. Contact any brewer, and ask if they have a hydrometer (what most use to check the sp), and I'd bet even money that all brewer will have one on hand. Adding salt flattens beer, so they're not checking salinity. I don't know what sp brewer's look for--but will find out if there's interest.
 
Too right. Before fermentation a best bitter has a specific gravity of about 1.045. We call it an "Original gravity" because of course it drops as the sugar ferments.

Beer wort is definitely not brackish! Water is brackish not because of a particular SG, but because it has reached a higher than FW SG through addition of seawater.
 
nattybrack, if you run a www.google.com search "specific+gravity sea water" as I just did, you'll get a range of values mostly ~1.002-1.003. Sea water varies. The Gulf of Finland has water so diluted with icemelt it tastes like flat Alka-Seltzer-- and cold as a martini! The Red Sea might have the high value you quoted.

There's so much besides salt NaCl in seawater--look at the ingredients on a marine salt mix. And as RTR noted, surprisingly little common salt in the alkaline Rift Lakes.

Here in New York we have some of the world's softest city water. They add polyphosphates to line the old pipes. But they don't boost our alkalinity, thank goodness.
 
Oh.. I got ya. Very cool. So it is not just the NaCl that makes the water brackish but all the minerals as well. Now, if you add the product, instant ocean, to feshwater, you can in fact make it brackish, because this product has the nec. minerals. But on the other hand, if you only add a small amount of instant ocean you will have, what is considered to be, a freshwater tank.
?
 
You must log in or register to reply here.
AquariaCentral.com
Top Bottom