There are many types of metal ions that can be found in pool water. By far, the most common metals found in water are ferric iron (Fe2+) and copper (both Cu+ and Cu2+). Metals are a problem in pool water because the ions are not stable or only mildly stable in water solutions at normal pool pH (7.0-8.0). Chlorine is a powerful oxidizer and it will react with metal ions forming oxidized metal that will drop out of solution. The conditions that are most unfavorable for metal ions are both high pH and high FC concentrations. Once these conditions occur, the metal ions will no longer stay in solution with water but will oxidize and fall out of solution forming metal scale and stains in your pool. Metal staining is difficult and expensive to correct and remove especially in the case of copper if the stains get old. The easiest rule of thumb for metal concentrations is you want them to be as close to 0 as possible. Typically speaking, iron in concentrations greater than 0.3ppm will start to scale out of solution and form brown water (tea colored) or yellow stains on pool surfaces. As well, copper ion concentrations above 3ppm will start to scale out of solution and form green colored water and black/brown stains on pool surfaces. Plaster pool surfaces are much more susceptible to staining because of the higher pH near the plaster surface while fiberglass steps in vinyl pools often show early signs of staining.
One point to note is that, in recent years, pool builders and equipment manufacturers have been pushing “low chlorine / alternative sanitizer” systems fairly heavily in lieu of traditional chlorine tablet feeders and salt water chlorine generators. These systems still require low levels of chlorine in order to achieve the EPA set standards for sanitation but also pair metal ion systems (Cu or Cu+Ag) with UV lamps and Ozone injectors. These systems intentionally inject metal ions into pool water as copper metal is an effective algaecide. However, the threshold levels of copper needed are so close to the scaling concentrations that these systems put pool water and surfaces at constant risk of staining. It is TFP’s recommendation that these metal ion systems be avoided at all costs as they are most definitely NOT trouble-free.
Metals can also be inadvertently added to the pool water by pool owners. Metal ions were often found in many different commercially available algaecides but, more recently, have been added to many types of “multipurpose” chlorine tablets. These tablets often contain only partial amounts of trichlor (a chlorine source) but often have sizable fractions of baking soda, metal salts (like copper citrate or copper sulfate) and weak clarifiers in them. Pool owners are fooled