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If you have a high TDS, it basically means that there is more dissolved minerals than there should be. An example of a mineral that will raise TDS is any of the nitrates, and as you know, nitrates are toxic at high concentrations to shrimp. Another is NaCl, table salt, again, at high concentrations, toxic to shrimp. Magnesium Sulphate is another, high concentrations means very hard water, makes it hard for shrimp to moult. Calcium Sulphate, same thing. Similarly, a TDS too low means there are a lot less dissolved minerals than there should be. The major issue with this is Calcium and Magnesium which the shrimp need to moult.