Should I Drain My Pool To Get Rid Of Algae?

When It Makes Sense

• Good fit: You have a severe black algae infestation that is embedded in plaster, pebble, or concrete surfaces. Black algae form protective roots and can survive normal shock levels, so draining may allow thorough scrubbing or acid washing of the exposed surface.
• Good fit: Your water chemistry is locked up with very high cyanuric acid or total dissolved solids, making chlorine ineffective. A partial or full drain and refill can reset the water so sanitizers work again, which helps prevent algae from returning immediately.

When You Should Avoid It

• Warning sign: You have an in-ground pool and the ground is saturated from recent rain or a high water table. An empty in-ground pool can float or pop out of the ground due to hydrostatic pressure, causing serious structural damage.
• Warning sign: You have a vinyl-liner pool. Liners can shrink, wrinkle, tear, or shift when the water is removed, especially in heat, and may be costly to repair or replace.

Pros and Cons

Pros

• Draining lets you remove heavily contaminated water and physically scrub or acid-wash surfaces that chemicals alone cannot fully clean.
• It resets stubborn chemistry problems, such as excessive stabilizer or metals, so future algae treatments have a better chance of working.

Cons

• There is a real risk of structural damage, including pool popping, liner failure, or cracking, particularly if drainage is done incorrectly or in wet soil conditions.
• It wastes thousands of gallons of water, costs money to refill, and requires time and chemicals to rebalance the fresh water before it is safe to swim again.

Decision Checklist

• What type of algae do I have, and how severe is the infestation? Green and mustard algae usually respond to shock and algaecide; black algae often needs stronger intervention.
• Is my pool safe to drain based on its construction, liner type, and current groundwater or soil moisture conditions?
• Have I completed a full chemical treatment cycle, including brushing, shock, filtration, and water balance, before deciding to drain?

Alternatives to Consider

Before draining, try a complete algae treatment: brush the walls and floor, shock the pool to a high free-chlorine level, add an appropriate algaecide, and run the filter continuously until the water clears. Test and balance pH, alkalinity, and cyanuric acid so chlorine remains effective. If stabilizer is too high, a partial drain and refill of roughly one-third to one-half of the water may restore chemistry without the risks of a full drain. Phosphate removers can also help by removing an algae food source. A pool professional can perform an acid wash or pressure wash without fully draining in some cases.

Final Recommendation

For typical green or mustard algae, draining is usually unnecessary and more risky than treating the water chemically. Reserve draining for severe black algae, damaged or deeply stained surfaces, or water chemistry that cannot be corrected with partial replacement. If your pool is in-ground, has a vinyl liner, or the ground is wet, consult a licensed pool professional before draining. This guide covers common situations, but structural and water-quality decisions can be high-stakes, so expert advice is worth the cost.