Water Ph And Tds

Sep 25, 2012
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I have a question regard the water quality, in particular the PH and TDS.
I am well were of the difference between the two, but I thought the TDS was low accordingly the PH is low. This was my understanding because when I spoke to the discus breeder they touch about PH, if I speak to pleco breeders they speak about TDS.
I have been testing regularly the water and to my surprise I find that the ph Of 5.5 has got a TDS of 740. Can some explain to me in few wards witch is the relationship between the two if any. Thank you


Feb 25, 2009
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TDS (total dissolved solids) is not in itself relevant to the pH.  TDS are all the dissolved solids occurring in given water.  The GH (mineral hardness) is part of the TDS, but there is much more.  I happened to have researched this for an article on another forum a couple years back, so to better answer your question I will cut/paste from that article and then comment on pH after.  Some of this may already be known to you, but it is generally better to provide all rather than too little information.
Total Solids basically refers to organic and inorganic matter that is either suspended or dissolved in the aquarium water.
Total Suspended Solids (TSS) refers to the amount of solid waste, decaying fish and plant matter, etc. that can be captured and held by a filter.
Total Dissolved Solids (TDS) is a measure of the combined content of all organic and inorganic substances contained in the water in molecular, ionized or micro-granular (colloidal sol) suspended form.  Generally the operational definition is that the solids must be small enough to survive filtration through a sieve the size of two micrometer.
Fresh water by definition contains no more than 1500 mg/l of TDS.  Brackish water contains 1500-5000 mg/l, and marine (salt) water has more than 5000 mg/l of TDS.  Note that mg/l is basically equal to parts per million (ppm), and also that this is not suggesting a level of 1500 ppm in an aquarium; these are just the approximate figures for the three categories.
TDS is connected to GH (general hardness) because like GH, TDS includes the calcium, magnesium and other “hard” mineral ions; these ions are what we measure with our GH test kits.  But water hardness correctly considered is more than this; both GH and KH can affect hardness and TDS levels; however, the reverse is not necessarily true.  Aquarium water can have a high TDS level but a low GH and KH (Jensen, 2009).  The TDS for instance also includes sodium (salt) ions, chemical substances, etc. which are not reflected in the GH.
TDS is basically everything dissolved in the water: chlorine, chloramine, ammonia, phosphate, salt, hard minerals (GH), bicarbonates (KH), etc.  And almost every substance added to the water will increase TDS: water conditioner, fish foods, plant fertilizers, calcareous substances, medications, water adjustment products, etc.
pH stands for pondus hydrogeni, Latin for “potential of hydrogen.” Water is made up of positively-charged hydrogen ions and negatively-charged hydroxyl ions, and pH is the measurement of the ratio of hydrogen and hydroxyl ions in a body of water. Acidic water contains more hydrogen ions, and basic (alkaline) water more hydroxyl ions; neutral water has an equal proportion. The pH is closely linked with the level of carbon dioxide (CO2) because CO2 produces carbonic acid. The hardness also impacts pH, since the carbonates bind to acids as they appear; as mentioned previously, this buffering will prevent or limit changes in pH.
The TDS affects fish, in some ways even more, than pH.  This is why water changes are so crucial, they and only they can reduce the TDS.  Soft water fish (like the discus, and most plecos) are naturally significantly impacted by TDS.
I am not a chemist, but if the above hasn't answered you, feel free to ask.


Aug 8, 2015
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Of all the elements there are 2 basic classes.  Metals and non metals.
Non metals include Sulfer,iodine,chloring, phosphorous, oxygen, carbon, and nitrogen. Acids are mailing made by these elements.
Most elements are metals.  Some such a sodium, and potassium are strongly alkalin Other are less alkalin such as Calcium and magnesium.  Most others are weekly alkalin. These elements are called Bases.
Acids lower PH while alkalin metals rase PH.  When you have very clean water the Ph is dominated by the gases in the water such as carbon dioxide (a week acid in water) and the PH will be low.  If you add sodium, potassium, magnesium,and calcium the PH will go up and TDS will go up.    IF you have water with few metals and a lot of none metals the TDS reading may be high but and the PH low. Acids and Bases prefer to react with each other forming salts such as potassium nitrate, or sodium carbonate. The resulting compounds may have little to no effect on PH.  
Acids and Bases prefer to react with each other. The resulting compounds may have little to no effect on PH.  However they will raise the TDS reading.  TDS is simply a measure of the conductivity of the water to the flow of electricity.  Pure water with no gas, metals, or non metals in it has very low conductivity and have a zero TDS reading.  Add any other elements to the water and it will start to conduct.  The higher the conductivity the higher the TDS reading.  
So you can have High TDS with low or high PH. Or you can have high TDS and neutral PH. You can also have Low TDS and a high or low PH. It all depends on the ratio of bases to acids and how strong each is and how much PH neutral stuff is in the water. 
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