Rust Spots

- there are a number of reasons why rust appears in a pool; its source could be from: -

Mains water

If the mains water to your property contains iron you will probably notice staining, in a small way, on other plumbing fittings around the house but it's dissolved in the water and is largely invisible until it becomes caught up in the pores of  lime-scale, gets exposed to atmospheric oxygen dissolved in the water - and rusts

Well water - there may be iron in the ground water, there may have been some iron fittings used in the construction of the well, or there may be some metal junk at the bottom of the well

Iron/steel components of pool equipment

Not very likely these days, as almost no iron or steel is used in the manufacture of pool-equipment, but some older pumps have a cast-iron body, or even iron pipe-work in old pools, and these can corrode; putting iron into the pool

Angle-grinder usage

If one has been used to cut stone, concrete or, particularly, steel near the pool the sparks given off will settle on the bottom of the pool and corrode. Be very careful about using an angle-grinder near a pool

Steel from the concrete structure of the pool

If the pool has been built from Gunite (or poured concrete) it's possible that one or more of the reinforcing steel bars was placed too close to the surface of the concrete. If this is the case the bars may corrode. As the bars corrode they expand, causing small cracks and fractures within the concrete, allowing more oxygen-bearing water to contact the steel and thus the corrosion accelerates


It's quite common for people to throw coins into a pool for kids to dive for but some modern coins will corrode in chlorinated water if left lying on the pool bottom - leaving behind unsightly rust-spots

Inappropriate screws

used to install lights, drain-covers, etc., screws should always be stainless steel or bronze - otherwise they will corrode

In the pool iron is reacted upon by Chlorine, corroding it and making it visible as a brown stain

Removal of iron from the water: -

Add a Chelating Agent, also sometimes known as a 'Sequestrant' although the two chemicals are distinct from each other and work in slightly different ways. These chemicals form a stable complex compound with metal ions and 'lock them up'.   Most commonly used are a group of chemicals, usually applied as a liquid, known as 'Amino Polycarboxolic Acids'. These chemicals wrap metal ions in a non-reactive coating. Reaction time is very quick and is typically measured in seconds, not days! 

Chelators have a preference for one metal ion over another - commonly they 'lock up' metals in this order: Iron, Copper, Manganese, Calcium, Magnesium, although there are Chelators that work on Calcium first

Sequestrants, more powerful stain removers than Chelators, react with metal ions in a slightly different way. They usually have several active sites on every molecule, each one attractive to a different metal ion, and can therefore lock away more than one metal at a time. Hence Sequestrants can control, for example, both iron AND copper in a pool

Rust Spot Removal

Ridding the water of iron is essential top prevent further staining but existing rust-spots on the walls or floor will not be removed immediately by either a Sequestrant or a Chelator. They must be dealt with independently of the root cause of the problem.

There are several proprietary chemicals, available from a pool store, all of which are effective to one degree or another.  Hydrochloric acid can also be used to etch away severely rust-stained plaster, but clearly this causes damage to the plaster and so should be used as a last resort

Some stains respond well to being treated with granular chlorine; sprinkle it onto the stain with the pump off. This method is a little more tricky if the stains are on the sides of the pool. I this case, pour the granulated Chlorine into a sock and hang it in the pool so that it is in contact with the stain

Your local pharmacy will certainly sell Vitamin C tablets and they can be used to remove rust stains. Applied directly to the stain, this treatment quickly removes staining without abrasion.  If you don't have Vitamin C tablets to hand you can use half a lemon - it takes a bit longer but it also works well 

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Ken Walker - MyPoolGuru©