Key takeaways
- TDS (total dissolved solids) measures the sum of all dissolved material in the water. Most residential pools should stay below 1,500 to 2,000 ppm above the TDS of the fill water source.
- In Austin, fill water TDS is already moderate to high from the tap because Central Texas groundwater is mineral-rich. That starting point leaves less headroom before pool TDS becomes problematic.
- CYA (cyanuric acid / stabilizer), calcium, salt (in salt pools), and mineral byproducts of chemical treatments are the main contributors to rising pool TDS.
- Evaporation concentrates everything dissolved in the water. Austin's hot summers can remove several inches of water per week, leaving minerals and chemicals behind in a smaller volume.
- A partial drain and refill dilutes TDS and other accumulated compounds. A full drain of a plaster pool without professional oversight risks plaster lifting or cracking.
- Austin-area drought conditions and water restrictions affect when and how much water can be added or replaced. Always check current conditions before planning a drain.
Austin context
Austin & West Austin tap water comes primarily from the Colorado River via the Highland Lakes system. It is moderately hard with a calcium hardness level that varies seasonally but is typically in the range where pool owners need to monitor calcium closely from the start, before any pool chemicals are added. When you combine that starting mineral load with Austin's average summer evaporation rate, which can be significant during hot dry stretches, the water concentration effect is meaningful. A pool that loses several inches per week and is topped off with fresh hard water keeps the pool level stable but does nothing to reduce dissolved mineral concentration.
Drought declarations and Stage restrictions on outdoor water use in Austin occur in dry years and can limit when pool owners are allowed to add water or perform a partial drain and refill. Austin Pool Bros monitors local conditions and can advise on timing a drain relative to active restrictions. In drought years, a planned partial drain followed by a refill may need to be scheduled during a restriction-free window rather than done on a routine timeline. The goal is to stay compliant while addressing a genuine water-quality problem.
What TDS measures and why it matters
TDS is a single number that represents the total amount of dissolved material in the water: calcium, magnesium, sodium, chloride, sulfate, carbonates, stabilizer compounds, and anything else that dissolves and stays dissolved. It is measured in parts per million (ppm) and is typically read with a TDS meter or a comprehensive water test.
High TDS affects pool chemistry in a few ways. Dissolved compounds compete with each other, reducing the effectiveness of sanitizers and pH adjusters. Water with high TDS can look dull or cloudy even when the core chemistry numbers look acceptable. In salt pools, an already-elevated TDS makes it harder for the salt cell to operate efficiently because the salt itself is part of the TDS count.
There is no single universal TDS threshold that triggers a drain. The generally accepted guideline for freshwater pools is to act when TDS rises more than 1,500 to 2,000 ppm above the starting TDS of the fill water source. For salt pools the discussion is more nuanced because salt intentionally raises TDS.
- Test TDS annually as part of a comprehensive water analysis.
- Know your fill water's starting TDS (your water utility publishes an annual water quality report).
- High TDS alone does not mean the pool is unsafe, but it does mean chemistry adjustments work less efficiently.
What drives TDS up in Austin pools
CYA (cyanuric acid, or stabilizer) is one of the largest contributors over time. Every tablet-fed chlorine system adds CYA with every puck because trichlor tablets contain stabilizer. Liquid chlorine and calcium hypochlorite do not add CYA, but most Austin pools use tablets at some point. Once CYA is in the water it does not break down; the only way to reduce it is dilution.
In salt pools, the salt itself is part of TDS. Salt pools typically run at 2,700 to 3,200 ppm salinity, which immediately adds several thousand ppm to TDS. A salt pool's total TDS is therefore higher baseline than an equivalent freshwater pool, which is normal and expected.
Calcium hypochlorite (cal-hypo) shock adds calcium with every dose. Combined with Austin's already hard fill water, repeated cal-hypo shocks can push calcium hardness toward scale-forming territory over a season.
Evaporation is the silent amplifier. When water evaporates, only pure water leaves; everything dissolved stays behind in a smaller volume. A pool that loses a significant amount of water to evaporation in summer and is continuously topped off becomes progressively more concentrated, even if no unusual chemicals are added.
- Trichlor tablet feeders add CYA with every puck.
- Cal-hypo shock adds calcium. Salt adds to baseline TDS.
- Evaporation concentrates all dissolved compounds. Topping off does not dilute.
When a partial drain and refill makes sense
A partial drain is appropriate when one or more compounds have accumulated beyond the range where chemical adjustment alone can fix them. The most common triggers in Austin pools are: CYA above 90 to 100 ppm (at which point the chlorine-blocking effect is significant), calcium hardness above 500 to 600 ppm (scale risk climbs sharply), or TDS elevated enough that chemistry corrections are not holding the way they should.
Partial drains, typically draining 25 to 50 percent of the pool volume and refilling with fresh water, dilute all dissolved compounds proportionally. A single partial drain at the right time can reset CYA, calcium, and TDS to manageable levels without the risks of a full drain.
The timing of a partial drain matters in Austin. Late fall or early spring, when temperatures are mild and the pool is not in heavy use, is generally the best window. Draining in peak summer is harder on plaster and leaves the pool exposed to UV and heat stress.
Why a full drain of a plaster pool requires professional oversight
Plaster is under compressive stress when the pool is full of water. When a plaster pool is completely drained, that pressure is removed, and the shell is exposed to temperature swings, direct sunlight, and soil pressure from the outside. In Austin's clay-heavy soils (which expand and contract with moisture), a completely empty shell can shift or lift if it is not properly managed.
Full drains of plaster pools should involve a qualified pool professional who can assess the shell condition, manage the drain rate, refill quickly, and make any necessary plaster repairs while the pool is empty. This is not a DIY project, and it is a decision that should be made based on water-quality data, not just a hunch that the water looks old.
Fiberglass and vinyl liner pools have different rules for empty pool safety, but any full drain should be discussed with a pool professional who knows your specific pool and soil conditions.
- Never drain a plaster pool completely without professional oversight.
- Austin's clay soils can shift an empty shell more than sandy or loam soils elsewhere.
- A partial drain of 25 to 50 percent avoids most plaster safety concerns.
Evaporation, concentration, and fill water math
Austin's summer evaporation can be significant, varying by pool size, surface area, sun exposure, and whether a cover is used. Larger surface area pools in full sun lose more water per week than smaller or shaded pools. Consistent topping off keeps the water level right but progressively concentrates dissolved minerals.
A useful way to think about it: if a pool loses 10 percent of its volume to evaporation per month and is topped off with fresh water, nothing is being diluted. The fresh water simply replaces the pure water that left. Over a full season, this can meaningfully raise the concentration of CYA, calcium, and TDS compared to where they started in spring.
A solar cover significantly reduces evaporation and, over a summer, also reduces the mineral concentration effect. If a cover is used consistently, the frequency of partial drains needed for TDS or CYA control decreases.
Austin drought conditions and water restriction caveats
Austin Water publishes a Water Conservation Stage status that can restrict outdoor water use during declared drought events. Restrictions can apply to pool filling and top-off, not just irrigation. Before planning a partial drain and refill, confirm the current stage and any applicable pool-filling rules.
In a year with significant Stage restrictions, the practical approach may be to accept slightly elevated TDS until a restriction-free window opens, then perform the partial drain. Maintaining correct sanitizer and pH during that period is more important than hitting a TDS target. A pool with elevated TDS but correct chlorine and pH is safer than one where chemistry has been neglected.
- Check Austin Water's current Conservation Stage before scheduling a drain and refill.
- During Stage restrictions, pool topping-off may be limited or time-restricted.
- Maintaining correct chlorine and pH is the priority if a refill must wait.
What a partial drain actually resolves
A partial drain at the right time resets CYA to a range where stabilizer is still protective but not chlorine-blocking. It drops calcium hardness to reduce scale risk. It lowers overall TDS so that chemical adjustments work more predictably. And in a pool where phosphate levels have crept up from organic debris, it provides a meaningful dilution of that as well.
A partial drain does not fix mechanical equipment problems, cannot remove algae that is already established on surfaces, and does not substitute for correct ongoing chemistry management. It is a water-quality reset, not a maintenance substitute.
Common mistakes
- Draining a plaster pool completely without professional oversight, risking plaster lifting or shell movement in Austin's expansive clay soils.
- Topping off the pool indefinitely without realizing that fill water adds minerals without diluting what is already in the pool.
- Ignoring CYA levels until they exceed 100 ppm or more, at which point a significant portion of the pool volume must be replaced to bring it back into range.
- Scheduling a drain during Stage water restrictions without checking current Austin Water rules.
- Assuming that adding fresh water during heavy evaporation periods is diluting dissolved compounds. It is not; it only replaces the evaporated volume.
Safe for owners
- Testing TDS, CYA, and calcium hardness annually with a comprehensive water test.
- Monitoring Austin Water's current Conservation Stage before adding water or planning a drain.
- Covering the pool to reduce evaporation and slow TDS concentration.
- Choosing liquid chlorine or non-CYA shock when CYA is already at the high end of the range.
Call a pro
- Planning and executing a partial drain safely, including managing refill timing.
- Any full drain of a plaster pool: assessing the shell, managing drain rate, and overseeing refill.
- Diagnosing whether high TDS, high CYA, or high calcium is causing a specific chemistry issue.
- Recommending the correct drain percentage based on current water chemistry readings.
- Evaluating whether a drain-and-refill or a targeted chemical treatment (such as a CYA reducer) is more appropriate for the specific situation.
Why weekly care matters
Weekly water testing catches rising CYA, calcium, and TDS trends early, when a modest partial drain can correct the problem. Austin Pool Bros tracks these numbers over time and can identify when the water is approaching the range where a partial drain will prevent chemistry management from becoming increasingly difficult. Catching the trend at 70 ppm CYA costs far less water and effort than correcting it at 120 ppm.
Common questions
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