Minnesota's Pool & Spa Experts · 763-542-9000 · Plymouth, MN

Spring Pool Care

Why Is My Pool Water Green After Winter? (And How to Fix It)

📅 April 20, 2026 · ⏱ 5 min read · ✍️ Dolphin Pool & Spa Team

Green pool water in a Minnesota backyard pool at spring opening

Images are AI generated and do not depict an actual pool or Dolphin Pool & Spa location.

You pull off the cover and your stomach sinks a little. The water is green. Maybe light green and hazy, maybe dark green and murky, maybe so dark you can't see the bottom at all. It's one of the most common spring discoveries for Minnesota pool owners — and while it looks alarming, green water is almost always fixable. Here's why it happens, how to read how bad it is, and exactly what to do to go from swamp to sparkling as fast as possible.

Pro Tip from Dolphin Pool & Spa

Don't waste money buying a cart full of chemicals before you know what you're dealing with. Bring a water sample into our store at 3405 Hwy 169 N in Plymouth and we'll run a full analysis — free with $50 in chemical purchases. We'll tell you exactly which BioGuard products you need and in what quantities for your pool size. Guessing at green water treatment costs more time and money than doing it right the first time.

1 Why does pool water turn green over winter?

Green pool water is almost always caused by algae — specifically green algae, the most common variety. Algae spores are naturally present in the environment and find their way into every pool. Under normal conditions, your chlorine keeps them in check. Over winter, that protection disappears and algae gets a long runway to establish itself.

Here's what contributes to a green pool at opening:

Chlorine dropped to zero over winter with no sanitizer present
Holes or tears in the cover allowed sunlight in — algae needs light to grow
The pool wasn't properly shocked or balanced at closing last fall

Warm late-fall or early-spring temperatures gave algae a head start
Debris accumulated under the cover, providing nutrients for algae growth
Water chemistry was significantly out of balance going into winter

2 Read your green — not all green pools are the same

The shade of green tells you a lot about how established the algae is and how long the recovery will take. Here's how to read what you're looking at:

Water Appearance	What It Means	Expected Recovery
Light green or teal tint, still somewhat clear	Early algae bloom, not yet established	1–2 days with proper treatment
Medium green, murky, can see 1–2 feet down	Moderate algae bloom, well established	3–5 days with aggressive treatment
Dark green, cannot see the bottom at all	Heavy algae bloom, likely months in the making	5–7+ days, may need multiple treatments
Black or very dark with visible wall growth	Black algae present — much harder to treat	1–2+ weeks, call us for a treatment plan

Heads up: If you can't see the bottom of your pool, do not let anyone swim in it — even partially. You cannot verify the condition of the drain covers or the pool floor, and heavily contaminated water is a health hazard. Get the water clear before anyone gets in.

3 The green pool treatment plan — step by step

Work through these steps in order. Skipping ahead or doing them out of sequence slows down recovery and wastes product.

Step A — Get the filter running

Remove the cover, reconnect all equipment, and get the filter running continuously. You will run the filter 24 hours a day throughout the entire recovery process — not just 8 hours. Dead algae and debris can only leave the pool through the filter. Clean or backwash the filter before you start and check it daily throughout treatment — a clogged filter dramatically slows recovery.

Step B — Brush everything

Before adding any chemicals, brush every surface of the pool — walls, floor, steps, corners, and around fittings. Brushing breaks up algae colonies and disrupts the biofilm that protects them. This makes your shock treatment dramatically more effective. Use a stiff-bristled pool brush for plaster or vinyl — not a wire brush on vinyl liners.

Step C — Balance pH first

Before shocking, get your pH into the 7.2–7.6 range. Shock is significantly less effective at high pH — if your pH is at 8.0, you're wasting most of your shock product. Use BioGuard Balance Pak 200 to raise pH or BioGuard Lo 'N Slo to lower it. With BioGuard's 15-minute re-dose window you can get pH dialed in quickly before moving on.

Step D — Shock aggressively

This is the most important step. For a green pool you need to super-chlorinate — use significantly more shock than the standard maintenance dose. Our recommendation by water color:

Water Color	BioGuard Product	Dose
Light green	Smart Shock or Burn Out 73	Double the standard label dose
Medium green	Smart Shock	Triple the standard label dose
Dark green or black	Smart Shock	Quadruple dose or more — come see us for a custom plan

Add shock at dusk or at night, broadcasting it evenly across the pool surface with the filter running. Run the filter overnight. The water may look worse before it looks better — turning grey or cloudy after shocking is normal and actually means it's working.

Step E — Add an algaecide

After shocking, add BioGuard Algae Complete or BioGuard Back Up to help knock back remaining algae and prevent regrowth. Algaecide works alongside shock — it attacks algae through a different mechanism, making the combined treatment more effective than either product alone.

Step F — Vacuum to waste

As the water clears and dead algae settles to the bottom, vacuum the pool to waste — bypassing the filter entirely so dead algae doesn't recirculate back into the water. Refill the pool to operating level after vacuuming, then retest and make any chemistry corrections needed.

Step G — Repeat as needed

For moderate to heavy algae blooms, one shock treatment is rarely enough. Retest chlorine levels the next morning — if free chlorine has dropped to zero overnight, the algae consumed it and you need to shock again. Repeat the brushing, shocking, and filtering cycle until the water holds chlorine overnight and begins to clear.

4 Why is the water still green after shocking?

If your pool has been shocked and is still green after 24–48 hours, one of these is usually the cause:

pH was too high when shock was added — shock loses effectiveness above pH 7.8
Not enough shock was used for the severity of the bloom
The filter is clogged and not circulating properly

Cyanuric acid is too high — above 80 ppm it locks up chlorine and makes shock nearly ineffective
The algae is yellow or black algae — much more resistant than green algae and requires a specialized treatment approach
Phosphates in the water are feeding the algae faster than chlorine can kill it

If you've shocked twice and aren't seeing progress, bring a water sample in. We can identify what's holding the recovery back and put together a targeted treatment plan.

5 How to prevent green water next spring

The best green pool treatment is never needing it. Here's what makes the difference between opening to clear water and opening to a swamp:

Shock the pool thoroughly before closing in fall — use BioGuard Smart Shock
Add a winter algaecide at closing — BioGuard Algae Complete is formulated to last all winter
Balance all chemistry before closing, especially pH and alkalinity

Use a quality cover in good condition — a cover with holes lets in sunlight and promotes algae growth
Keep water level correct over winter so the cover doesn't sag and collect debris
Open the pool before temperatures consistently exceed 60°F — the earlier you get chemistry balanced, the less time algae has to grow

A green pool at opening is frustrating, but it's fixable — and it's preventable. With the right BioGuard products and a methodical approach, most green pools are swimmable within a week. The key is not to panic, not to guess, and not to add random chemicals hoping something works. Test, treat, circulate, repeat. And when you need a hand — we're right here.

Dolphin Pool & Spa Reminder

Dealing with a green pool and not making progress? Bring a water sample into our store at 3405 Hwy 169 N in Plymouth and we'll run a full analysis, identify what's blocking recovery, and put together a customized BioGuard treatment plan for your pool — free with $50 in chemical purchases. We've cleared thousands of green pools across the Twin Cities. We know what works.

Green pool? We can help.

Don't spend weeks guessing. Bring your water sample in or give us a call and we'll get you on the fastest path from green to clean. Dolphin Pool & Spa has been solving Minnesota pool problems since 1988 — green water is no match for a good treatment plan and the right BioGuard products.

📞 Call Us Now — 763-542-9000

Your Happy Dolphin Pool & Spa Team
Serving Minnesota pool owners since 1988 · 3405 Hwy 169 N, Plymouth, MN · 763-542-9000