By Donna M. Recktenwalt
Good water is essential to the successful maintenance and breeding of tropical fish. But how do we know if we have good water?
Water values can be measured several different ways, but in general, water can be measured for hardness (total and Carbonate), conductivity, and osmotic pressure. Also, water can be measured for hardness and for salinity, which should not be confused. The osmotic pressure on a fish’s surface is strongly affected by salinity. Oodimium and other flagellate infections are greatly hindered by the strengthened slime provided by added salt.
Hardness measures the amount of dissolved salts. Strictly, total dissolved solids represent the total amount of solids remaining when a water sample is evaporated to dryness. In practice, aquarium hobbyists don’t measure it that way. The meters that measure TDS (in ppm) measure the conductivity of the water.
Conductivity measures the water’s ability to transport electrical current. The conductivity is due mainly to the ions in solution, which is mainly a function of the dissolved solids. The more dissolved salts, the more ions, the more conductivity. Added salt will change the conductivity of the water. This measure provides a rough estimate of the total amount of dissolved salts.
Conductivity meters (that measure microSiemens) operate on the same principle as TDS meters. The essential difference between them is in the calibration of the readout. In general, in most fresh water the reading for conductivity in microSiemens will be double the value that one gets for TDS in ppm, thus Conductivity (microSiemens) = 2 x TDS (in ppm).
Hardness can also be measured using red-to-blue colorimetric test kits. These can provide fairly consistent results, with the color transition occurring over a several drop (of test reagent) range. Bruce Turner says that he uses a distilled water standard, along with the test water, and matches the deep blue color of the distilled water standard with his test. He does the resulting comparisons against a white index card in reflected light. By using tapwater and distilled water in 1:1, 1:2, 1:3 etc. proportions, he says that he can recover a fairly standard curve. Likewise, he can boil a sample of tapwater and let it stand for an hour, then compare it with water from the tap. Generally about 30% of the hardness disappears (temporary/carbonate hardness). NaCl doesn’t seem to interfere with the reaction.
Osmotoic pressure provides an estimate of the total amount of dissolved ions and nonions.
Examples:
1) If you mix regular table salt (NaCl) in water, you won’t change the hardness, but you will change the conductivity and the osmotic pressure. Regular table salt contains only parts per thousands of contamination, most of it insoluble.
2) If you mix in limestone you will change the hardness, the conductivity, and the osmotic pressure.
3) If you mix in sugar, you will change only the osmotic pressure. Plants have starch in their roots for pulling water in from their surroundings, via the law of equilibirum.
Experiments:
Purchase a gallon of distilled water.
1) Measure it for hardness, carbonate, total, etc.
2) Boil the water for three minutes, cool to room temperature, and remeasure the hardness.
3) To the boiled water add 1/4 tsp. salt per 2 cups water. Retest. There are dissolved solids in the water, but the hardness is zero. You made it that way.
4) Do the same with another 3-4 cups of water, boil and cool, test for hardness. Now add 1/4 tsp. Plaster of Paris. Stir to dissolve. This will produce total hardness, but no carbonate hardness. Retest.
– GCKA Newsletter, May 1999