Which Pool Chemicals Are Bad for My Pool? The 9 Chemicals That Damage Surfaces (2026 Guide)
Some pool chemicals quietly destroy fiberglass, marbelite and equipment. Here are the 9 chemicals and habits to avoid — written by Gauteng pool renovators…
We renovate over 200 Gauteng pools a year, and at least a third of the surface failures we see were caused — or accelerated — by the wrong chemicals or the wrong way of using the right chemicals. The damage is preventable. Here are the 9 chemicals and habits that damage pool surfaces, and what to use instead.
1. Trichlor (chlorine) tablets dropped into the pool
Trichlor tablets have a pH of around 2.8 — extremely acidic. When dropped directly into the pool or skimmer basket, they sit against the surface and bleach permanent white or pink spots into marbelite, fade fiberglass gel coat, and corrode metal fittings.
What to use instead: a floating tablet dispenser (cheapest option) or, ideally, an in-line chlorinator plumbed after the heater. Never drop tablets loose into the pool, ever.
2. Calcium hypochlorite (HTH) shock applied undissolved
Cal-hypo shock is excellent for sanitising, but if you scatter granules directly across the pool floor, undissolved granules sit on the surface and create permanent bleached craters in marbelite.
What to do instead: pre-dissolve granules in a clean bucket of pool water, then pour the solution slowly around the perimeter with the pump running.
3. Hydrochloric (muriatic) acid poured undiluted
Pouring concentrated HCl straight into the pool to lower pH causes severe localised etching of marbelite and gel coat damage on fiberglass. We see the streaks weekly.
What to do instead: dilute 1 part acid into 10 parts water in a plastic bucket (always acid into water, never water into acid), then add slowly to the deep end with the pump running. Or use dry acid (sodium bisulfate) which is safer and gentler.
4. Copper-based algaecides (long-term use)
Copper algaecides work, but copper accumulates in the water and eventually stains marbelite green-blue and fiberglass dark grey. Once metal staining is in the surface, only an acid wash or full resurface removes it.
What to use instead: polymer-based (PHMB) algaecides or simply maintain proper free chlorine (1–3 ppm) and weekly brushing. Use copper algaecide as a once-off rescue only, not a routine treatment.
5. Borehole water without metal sequestrant
Many Pretoria, Centurion and East Rand pools are topped up from boreholes. Borehole water is often loaded with iron and manganese, which oxidise on contact with chlorine and stain pool surfaces brown, black or green.
What to do instead: test borehole water before topping up. If iron is present, dose a metal sequestrant (e.g. HTH Stain & Scale or equivalent) before adding chlorine.
6. Salt-chlorinator pools with low stabiliser
Salt chlorinators are great, but if you let cyanuric acid (stabiliser) drop below 30 ppm, the chlorine produced is consumed by UV within hours and you'll dose the pool harder, accelerating surface wear.
What to do instead: maintain stabiliser at 30–50 ppm in salt pools, 30–80 ppm in tablet pools.
7. Allowing pH to drop below 7.0
Low pH is the single most common cause of premature marbelite failure in Gauteng. pH below 7.0 actively dissolves the cement matrix in marbelite and etches the gel coat on fiberglass.
What to do instead: test pH weekly and hold it at 7.2–7.6. Dose with soda ash (sodium carbonate) to raise it.
8. Allowing calcium hardness to fall below 200 ppm
Soft water is hungry water — it pulls calcium out of marbelite, causing chalky white surfaces and rough patches. Common in pools refilled from rainwater tanks.
What to do instead: maintain calcium hardness at 200–400 ppm. Add calcium chloride if low.
9. Pool paint over a sound surface
Pool paint marketed as a 'cheap renovation' is the worst long-term decision a Gauteng pool owner can make. It blisters within 18 months under Highveld UV, and once it's on, every future renovation requires a full mechanical strip-back at significant extra cost.
What to do instead: if your pool surface is failing, get a proper marbelite or fiberglass quote. The cost-per-year is lower than repainting every 2 years.
Need help fixing chemical damage?
If your Gauteng pool already shows chemical damage — bleached spots, etched walls, metal staining or chalky surfaces — a free on-site assessment will tell you whether it's reversible with an acid wash or whether the surface needs renovating.
Call 079 932 8715 or WhatsApp us. We serve all of Gauteng with a written 5-year workmanship guarantee.
Need a quote for work in Gauteng? Contact Swift Pool Repairs or see our pool renovation services.
Free quote: 079 932 8715 · info@swiftsolar.co.za · Serving all of Gauteng.