Skip to main content

 

Dolphin Pool & Spa

BioGuard Erase: Rescuing a Pool With Iron or Rust Stains Mid-Season

August 6, 2026

Pool technician treating rust stains on pool wall

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

August heat and heavy pool use bring a surprise for a lot of Minnesota pool owners: reddish-brown or blue-green stains on the walls and floor. Metal stains show up fast this time of year, often right after a well-water top-off or a heavy summer rain. The good news is you can fix this without draining the pool.

1 Know What You're Looking At

Metal stains and algae look similar at a glance, but they act different. Here's how to tell them apart.

Metal Stains

  • Rust-brown, blue-green, or gray-black color
  • Shows up on walls, steps, and around fittings
  • Won't brush away completely
  • Often follows a fill-up or heavy rain

Algae

  • Green, yellow, or black patches
  • Feels slimy to the touch
  • Brushes away, then returns fast
  • Gets worse in warm, still water

Quick home test: Drop a vitamin C tablet on the stain underwater. If the stain fades where the tablet touches, it's a metal stain, not algae.

2 Test Your Water Before You Treat

Before you touch BioGuard Erase, your chlorine level needs to read 0–1 ppm. Stop in for a water test — it's $7, or included with a $50 purchase of chemicals — and we'll check your levels and confirm you're ready to treat.

Heads up: Skip this step and BioGuard Erase won't work right. High chlorine breaks down the ingredient that pulls stains out of your plaster.

3 Apply BioGuard Erase the Right Way

BioGuard Erase is a granular product. Broadcast it evenly over the stained areas, brush it in, and keep your pump and filter running per the label directions. Give it time to work before you judge the results.

Heads up: Do not shock your pool for four days after applying BioGuard Erase. Shock too soon and the stain can come right back.

4 Lock In Results With Pool Magnet Plus

Once the stain is gone, add BioGuard Pool Magnet Plus to keep it that way. This product holds iron, copper, and manganese in your water so they can't bind to your plaster again. Add it weekly, especially after a fresh fill or a heavy August rain washes new metal into your water.

5 Get Back to Your Routine Fast

Once your four-day wait is up, get back to your normal chlorine routine. This is where BioGuard pays off: generic chemicals need up to four hours between doses. BioGuard products need just 15 minutes. After a stain treatment, that means you spend less time waiting and more time swimming.

Step What To Do Wait Time
Test water Chlorine must read 0–1 ppm None
Apply BioGuard Erase Broadcast granular, brush in, run filter Per label directions
Hold off on shock Skip shock treatments 4 days
Add Pool Magnet Plus Weekly dose to keep stains from returning Ongoing

Rust and metal stains are common in Minnesota pools this time of year, especially after a heavy summer rain. Stop by Dolphin Pool & Spa for a water test — $7, or free with a $50 chemical purchase — and we'll help you pick the right products to fix it fast. From testing to treatment, we make pool ownership a joy!

Call Us Now — 763-542-9000

Your Happy Dolphin Pool & Spa Team
Plymouth, MN