2Hr Aquarist Commentary: TDS meters such as this model from HoneForest are useful for quick TDS readings. TDS readings do not tell you the content of the water, it merely tells you the total amount of dissolved solids present (even that it is an approximate reading). It is useful to check for relative changes in value - the absolute reading is less useful. For example, if your tap water tests consistently at 50ppm TDS, the day that you test it and the reading goes up to 100ppm you know that something has changed. The TDS test does not tell you what changed though. To that end, planted tanks can work in a very wide range. Tanks grow perfectly well from 10ppm TDS all the way upwards of 500ppm TDS. What makes up the TDS (which can only be discerned through other test kits) is infinitely more important than the absolute value itself.
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