By Donna M. Recktenwalt
Killiekeepers have many tools available to help monitor water quality, including test kits of various kinds. One such tool measures the total dissolved solids (TDS) in water. Readouts from such a unit are useful in measuring the relative TDS, and fish seem to be more sensitive to major differences in hardness (dissolved solids) than they do to differences in pH.
Scheel, in his chapter dealing with breeding non-annuals in Rivulins of the Old World, wrote: “As my two types of water have similar salinities I can move the individuals right in and right out of one type of water without any harm. Differences in pH values, even from 5.0 to 8.0, have not produced any harm when individuals are moved from one type of water to the other.” This 3 pH unit jump equals a 1000x change in acid-base concentration.
“My impression (based on experience and lots of mistakes) is that osmocity (TDS change), and not acidity (pH change) is the stronger stress-producing change agent,” says Wright Huntley. “Somehow, going quickly from high TDS to low is worst. Apparently cells start to explode before the osmotic barrier has a chance to form up. That seems to cause more permanent gill damage than the cell dehydration of going from soft to hard water. When in doubt, drip acclimate.”
Additionally, “As fish are repeatedly bred in captivity, they usually lose the need for soft acid water to do well. Many rain-forest fishes adapt over the generations to the harder, more alkaline water [of aquaria].”
“There are some practical aspects [about TDS meters] that should be kept in mind,” points out Barry Cooper.
1. The small TDS and conductivity (MicroSiemens) meters both measure the same thing, i.e., conductivity. They are simply calibrated differently, so that one produces a reading numerically twice the other.
2. The only way to determine true TDS is to evaporate the sample to dryness and weigh the solid residue.
“I would bet,” adds Barry, “that (by weight) the vast majority of dissolved material in the average pool or tank water is mineral matter (which produces most of the charged ions) rather than organics, etc.” In any event, “the TDS meters that hobbyists use measure conductivity and dissolved organics do not add to that.”
TDS meters “measure what the fish feel, pretty well,” says Wright Huntley. “The TDS determines how the fishes’ three-level fluid control system works to maintain body fluids by osmosis. In my opinion, sudden changes in TDS are dramatically more harmful that pH ‘shock.’ “
“The other factor that needs to be considered is the effect of those dissolved solids on colligative (chemical binding) properties such as osmotic pressure, boiling point, elevation, freezing point etc.,” Lee Harper points out. “Osmotic pressure is perhaps the most important, because that is the property that will destroy gills if changed too rapidly, especially downward. Any material in solution will affect the colligative properties of the solution, whether ionic or nonionic, organic or inorganic.” Very simply put, “the property of the dissolved material that affects the colligative property is the number of atoms or molecules.” Thus, “a trisodium salt (trisodium phosphate, for example) will have approximately 3 times the effect on osmotic pressure as a mono sodium salt or an acid of the same molecular weight.”
“Remember these two principles and the consequences,” Lee recommends:
1. Change fish to a new environment gradually if you have any doubts about the water chemistry.
2. Be especially cautious in switching to water with lower dissolved solids, because that change is most damaging to the gills. To avoid unexpected problems add some salt (sodium chloride) to all water and the shock to new fish will be much less severe.
– G.C.K.A. Newsletter, September 1999