tried searching and all I get is tourism stuff and things about the size of the river, ect. Nothing about the salt.
and you won't. the amazon is an extremely complex river system and is extremely variable in its chemical composition. the Amazon drains an area of some 6,915,000km² (2,722,000 mile²), or some 40 percent of South America. It gathers its waters from 5 degrees north latitude to 20 degrees south latitude. the quantity of fresh water released to the Atlantic Ocean is enormous: 6.5 million cubic feet per second (184,000 m³/s) in the rainy season. the Amazon is responsible for a fifth of the total volume of fresh water entering the oceans worldwide. thus, depending on the drainage from which a sample was taken, the TDS and salinity would be very different.
in the amazon, we're talking like a tablespoon or so per 1,000,000 gallons
that's not even close to accurate. here's a 'chart' expressing the ionic concentration difference between the upper regions of the Amazon (Solimoes) and the lower regions (Rio Negro).
Solimoes -- Rio Negro
Na (mg/L) 2.3 ± 0.8 0.380 ± 0.124
K (mg/L) 0.9 ± 0.2 0.327 ± 0.107
Mg (mg/L) 1.1 ± 0.2 0.114 ± 0.035
Ca (mg/L) 7.2 ± 1.6 0.212 ± 0.066
Cl (mg/L) 3.1 ± 2.1 1.7 ± 0.7
Si (mg/L) 4.0 ± 0.9 2.0 ± 0.5
Sr (μg/L) 37.8 ± 8.8 3.6 ± 1.0
Ba (μg/L) 22.7 ± 5.9 8.1 ± 2.1
Al (μg/L) 44 ± 37 112 ± 29
Fe (μg/L) 109 ± 76 178 ± 58
Mn (μg/L) 5.9 ± 5.1 9.0 ± 2.4
Cu (μg/L) 2.4 ± 0.6 1.8 ± 0.5
Zn (μg/L) 3.2 ± 1.5 4.1 ± 1.8
Conductivity 57 ± 8 9 ± 2
pH 6.9 ± 0.4 5.1±0.6
Total P (μg/L) 105 ± 58 25 ± 17
Total C (mg/L) 13.5 ± 3.1 10.5 ± 1.3
HCO3-C (mg/L) 6.7 ± 0.8 1.7 ± 0.5
as far as adding salt to your "freshwater" aquarium to help regulate osmotic balance, this just isn't based on science and in fact, it goes against many of the principals of aquatic biology as we know them. All fish are different depending on where they live. Some species are native to "soft water/low dissolved solids" habitats while others inhabit hard and alkaline biotopes with high salt concentrations inherant in the water. The species inhabiting these very different environments have developed metabolic processes which take advantage of the specific chemistry of that water. To add salt in the misguided attempt to "help regulate osmotic balance" may actually upset that very balance you are erroneously trying to "help". In fact, your fish are much better served by leaving them alone and allowing them to regulate their own osmotic balance ... something they have been doing without your help for thousands of years.
How about the myth that says "salt is a tonic for fish when it is kept in fish tanks at moderate levels". rediculous. If your fish are the most commonly kept community fish such as tetras, corys, angels, rasboras, barbs and most anabantids, these are fish largely from soft, acid, low-TDS (total dissolved solids) waters. The average tap water in the U.S and Europe is at least moderately hard and alkaline and is certainly not improved by increasing the "salt" concentration, that very thing in which your water already differs most from the natural waters of these fish. Certainly many of these fish adapt well to our local water conditions and I am an advocate of adapting non-breeding fish to local conditions. This is far better for both fish and keeper than constantly battling see-sawing water parameters. However, acclimation to your tap water is one thing .... making your water worse than it already is however, is quite another.
Then there's the prophylactic use of salt to avoid common pathogens and parasites of fish by keeping some level of salt in the tank. More baloney. True, the use of salt, usually accompanied by increased temperature, is an effective treatment for one of the most common ectoparasite which beginning aquarists encounter, Ichthyophthirius multifiliis, or Ich. However, only the free-swimming form of this parasite is treatable. Neither the fish-embedded growth phase, nor the encysted multiplication stage that rests on or in the substrate is readily attacked by salt or any other medication. Thus prophylactic treatment is useless as it makes little sense to treat a non-infected tank on a chronic basis.
Another falacy is the addition of NaCl for livebearers. These fish as a group are native to estuarine environments where the waters are likely to be hard and alkaline at least and may even be brackish. The addition of sodium chloride (i.e, "salt") does little to match these species native waters since salt comprises only a portion of the total "salt" concentration. The more significant elements are calcium, magnesium, potassium, boron and silicate salts. Moreover, it's the total dissolved solids (TDS) that is important and not necessarily the 'salt' concentration therein. It's also important to note that the term "salt" isn't limited to sodium chloride. Calcium and Magnesium salts of carbonate, silicate, phosphate and borate are actually more important to the biology of brackish water fish.
If your water is at least moderately hard (GH and KH 8-12) then the addition of some NaCl to your water will do little harm. On the other hand if your water is soft and acid the addition of sodium salt may do significant harm. You need increased buffering capacity as much as the addition of minerals to the water and NaCl alone does absolutely nothing in this regard. Adding crushed coral or aragonite, both of which are primarily calcium carbonate salts, or marine salt mix which contains the whole range of cations and anions (positively and negatively charged ions, such as Na+, Ca ++; Cl-, CO3--) found in the sea can adequately buffer your water and more closely match the native waters of most live bearers in question.
Salt does have other limited and specific uses. It temporarily reduces the effect of nitrite toxicity at 0.1-0.3 % -- the chloride ion counteracting the nitrogen blockage of oxygen uptake. Salt is useful for the erradication of hydra at 0.3-0.5 % for five days. Salt will remove leeches from pond fish as a 3.0 % bath for 15 minutes. Salt will mitigate the affects of ulcer disease in cold water fish as a 1.0 % addition to the tank water by temporarily reducing osmotic stress.
Beyond these few 'uses of salt' ... it has no place in your freshwater aquarium.
