It is very difficult to make one definite diagnosis when faced with symptoms of swimming pool water problems. You should use other evidence when considering what to do (e.g. test results) and go with the one you think is most likely.
Symptom : Black Spots
Diagnosis

These are caused by black algae.
Cure

Unfortunately, black algae is very tenacious and will require quite a bit of effort to remove. For minor problems, try persistent brushing combined with a good black algaecide. You will also need to clean the filter thoroughly. If the problem is more major, you may need to drain the pool water and chlorine wash.
Symptom : Cloudy / Milky Discoloration
Diagnosis (1)

Dirt or bather pollution in water suggesting inadequate filtration
Cure (1)

Backwash the filter and raise free chlorine levels to around 10 parts per million using un-stabilized chlorine. Polish water by adding a clarifier.
Diagnosis (2)

You are using stabilized chlorine's and there is too much stabilizer in the water. This causes the chlorine to take longer to kill micro-organisms and so they build up and give a haziness to the water.
Cure (2)

Lower levels of stabilizer by replacing some of the water – either by performing an extra large backwash or by draining to waste. Then top up with fresh mains water. Super chlorinate by raising free chlorine levels to around 10ppm using un-stabilized chlorine.
Diagnosis (3)

The swimming pool filter is ineffective or has become blocked.
Cure (3)

Check the pool filter sand or media and replace if necessary. If the sand particles have become coated in calcium (this happens mainly in hard water areas), the filter is not blocked and everything else seems fine, sharpen the sand with a filter cleanser.
Diagnosis (4)

Fine suspended particles in water that are formed as dissolved hardness salts transform to small solid particles by a process called precipitation. This is probably a result of a high pool water pH or alkalinity.
Cure (4)

Lower the pH using reducer until an adequate reading is obtained.
Symptom : Cloudy Green
Diagnosis

Algae is present in the water. This suggests that chlorine levels have dropped too low or it has become ineffective.
Cure

Shock dose with chorine to kill the algae (10ppm for green tints, 25ppm if the bottom of the pool is not visible.) Brush off remaining algae on pool surface (including underwater lighting and step ladders.) Wait 24 hours. Backwash filter to clear out dead algae. Use water clarifier to remove haziness.
Prevention

Maintain chlorine levels at around 3 ppm. Use an algaecide (or chlorine with added algaecide) regularly if algae becomes a big problem.

If you are using stabilized chlorine and testing indicates adequate levels of chlorine but you are still getting algae, see Diagnosis 2 above
Symptom : Rusty red colored water
Diagnosis

Pipes or other steel/iron-containing fittings in the circulation system are being corroded by a high water acidity (low pH level.) The corroded particles are then oxidized by chlorine shock doses and this causes rust.
Cure

Act quickly to prevent the liner being stained and other damage. Contact us and see whether you can drain down and replace the water all at once or whether you need to use progressive dilution. Use a good liner and tile cleaner to remove any rust stains.
Prevention

Replace the offending metal fittings with PVC or copper. Ensure that the pool water has a pH and total alkalinity level within recommended limits.
Symptom : Sore throat, skin irritation and stinging eyes.
Diagnosis (1)

The pH level of the water is too high (alkaline) or too low (acidic).
Cure (1)

Aim for a pH of around 7.4. Use chemicals to lower the pH or to raise the pH.
Diagnosis (2)

High levels of combined chlorine (chloramines). An unpleasant "chlorine" smell tends to accompany this problem. Perform a DPD no3 test and check the results.
Cure (2)

Super chlorinate the pool (up to 10ppm free chlorine) in order to break down the chloramines.
Prevention (2)

Ensure that combined chlorine forms no more than 1/3 of the total chlorine in the pool.
Diagnosis (3)

You are using a detergent that is not chlorine compatible, for example to remove tide-marks or to clear pool sides. Some detergents react with the chlorine and these reactions can cause irritation to the skin and eyes. Traces of soaps and shampoos (from bathers) can cause similar reactions.

Super chlorinate to ensure that all of the detergent(s) has/have reacted away.
Prevention (3)

Switch to cleaners that are compatible with chlorine or clean manually.
Diagnosis (4)

You are using the pool whilst sweaty or wearing makeup. Perspiration and cosmetics both contain nitrogenous compounds which react with free chlorine.
Prevention (4)

Have a thorough wash or shower before entering the pool.
Symptom : Tinted or blonde hair turns a green color after swimming.
Diagnosis

Levels of copper in the pool are too high. It may be that the pool water is too acidic (low pH) and that this is corroding the fittings of the heater. Alternatively you may be over-using algaecides that are copper-based.
Cure

Your local pharmacist should be able to recommend a shampoo to restore the hair's original color
Prevention

Use copper-based algaecides more sparingly. Raise the pH of the pool water.