Why Dichlor Granules End Up on the Pool Floor

If you find dichlor granules sitting undissolved on the pool floor, stop and assess water chemistry, circulation, and granule condition before applying any further treatment. Proceeding without these checks risks localized over-chlorination, surface damage, and wasted product.

Dichlor granules (sodium dichloroisocyanurate, often abbreviated SDIC dihydrate or anhydrous depending on the product) are designed to dissolve gradually when distributed through the water column or a feeder. Granules that accumulate on the bottom usually indicate one of three problems: the circulation system is not moving water adequately, the granules were dumped directly onto the floor instead of pre-dissolved or fed slowly, or the water chemistry has shifted so far from the optimal range that dissolution has slowed or stalled. Each of these requires a different corrective step before you resume normal dosing.

Check 1: Confirm Whether the Granules Are Still Active or Have Degraded

Before you decide how to treat, look at the granules themselves. Intact, white-to-off-white dichlor granules that retain their shape have not yet released their available chlorine. If they appear clumped, sticky, or partially dissolved into a concentrated paste, they may be creating a high-chlorine micro-zone on the pool surface.

What to do: Physically remove any visible accumulation from the floor with a pool net or manual vacuum on the waste setting. Do not attempt to brush concentrated granules off a plaster, vinyl, or fiberglass surface with dry equipment—the undissolved product can bleach or etch the finish. Rinse the area with fresh water after removal, then check the surrounding water for elevated free chlorine before proceeding.

Check 2: Test the Current Water Chemistry

The most critical step is a full water test before you add more dichlor or switch to any other sanitiser. Key parameters include:

• Free chlorine: Verify the residual is within the target range for your pool type (typically 1–3 ppm for residential pools, up to 5 ppm for commercial or public pools under some codes). Undissolved granules on the floor can produce a false high reading if the test sample is drawn from near the deposit.
• pH: Dichlor is mildly acidic (a solution of sodium dichloroisocyanurate typically reads around 6.0–6.8). If the bulk pool pH is already below 7.2, adding more dichlor will push it lower, reducing sanitiser effectiveness and increasing corrosion risk. Target pH 7.2–7.6 for optimal chlorine activity.
• Cyanuric acid (CYA): Dichlor contributes CYA with every dose. If CYA has climbed above 50–80 ppm (the upper limit depends on local guidance), chlorine efficacy drops significantly regardless of how much you add. At that point, the granules on the floor are not your only problem—you need a partial drain and refill strategy, not more product.
• Calcium hardness and total alkalinity: Low calcium hardness combined with acidic dichlor can leach calcium from plaster or gunite surfaces, especially where granules sit. Total alkalinity below 80 ppm offers little buffering, making pH swings worse.

Take the test sample from mid-water, away from the floor deposit, to get a representative reading. Record all values before deciding the next dose or corrective action.

Check 3: Evaluate Circulation and Filtration

Dichlor granules rely on water movement to dissolve evenly. If the pump, filter, or return jets are not functioning, granules will sink and stay on the floor. Check the following:

• Is the pump running and primed? Listen for unusual noise or check the pressure gauge—a high reading suggests a clogged filter or blocked skimmer.
• Are return jets directing flow across the pool floor? Dead zones where water is stagnant allow granules to settle and concentrate.
• Is the filter media clean? Sand or DE filters that are overdue for backwashing reduce flow rate, which directly slows granule dissolution.

If circulation is compromised, fix it first. Adding more dichlor to stagnant water simply creates another layer of undissolved granules. Restore normal turnover—typically 6–8 hours for residential pools—before you consider any further chemical treatment.

Check 4: Inspect the Pool Surface Under the Deposit

Concentrated dichlor sitting on a surface for even a short period can cause bleaching or etching, particularly on plaster, pebble, or darker-colored vinyl. After removing the granules, examine the area:

• Look for lighter patches on plaster or aggregate finishes.
• Feel for any rough or chalky texture that was not present before.
• Note whether the damage is limited to where the granules sat or extends outward (which would indicate a broader water-balance issue).

If you observe surface damage, do not apply additional dichlor directly to that area. Allow the surface to recover and rebalance the water chemistry before resuming any treatment. This may require targeted pH adjustment, a partial water replacement, or consultation with a pool surface specialist—depending on the severity.

Check 5: Water Temperature and Its Effect on Dissolution

Dichlor dissolves more slowly in cold water. If your pool temperature is below 15 °C (59 °F) and you have been dosing granules on the floor expecting quick dissolution, the low temperature is likely the cause. In such conditions, pre-dissolving the granules in a bucket of warm pool water and pouring the solution around the return jets is a safer approach than floor application. Confirm the temperature before deciding whether the granules failed or the conditions prevented proper dissolution.

What Not to Do When You Find Dichlor Granules on the Floor

Avoid these common mistakes:

• Do not sweep or vacuum the granules into the main drain without first rinsing—undissolved chlorine in the pump strainer basket can damage seals and create hazardous gas pockets.
• Do not mix dichlor granules with any other pool chemical, including acid, bromine, or algaecides. The combination can release chlorine gas or cause an uncontrolled reaction.
• Do not assume the granules are “fine” because they look unchanged. Even intact granules on the floor represent a dosing error that will skew your water balance if you ignore it.

Deciding the Next Step

Once you have checked granule condition, water chemistry, circulation, surface condition, and temperature, you can determine whether to: (a) simply restart the pump and allow existing granules to dissolve, (b) remove the deposit and rebalance the water, or (c) adjust your dosing method (pre-dissolving or using a feeder) to prevent recurrence. The decision depends entirely on what those checks revealed—there is no single corrective action that fits every situation.

If you are unsure whether your current dichlor product formulation or particle size is suited to your pool’s operating conditions, contact ENVO CHEMICAL with your pool volume, current water test results, and a description of the granule behaviour you observed so we can discuss the appropriate next step.