Why Won’t My Hot Tub Stay Clean?

If your hot tub goes cloudy a day or two after every cleaning, the problem is usually not the chemistry you’re adding — it’s what’s already in the lines.

The Real Culprit: Biofilm

Hot tub plumbing is a long, warm, dark environment. That’s ideal for biofilm — a slimy layer of bacteria, dead skin cells, lotions, and other organics that coats the inside of pipes, jets, and the heater.

Biofilm constantly sheds back into the water. You shock the water clean. The film sheds. You’re cloudy again. You add more chlorine. The film resists chlorine because it’s protected inside the matrix. The cycle continues.

Signs You Have a Biofilm Problem

• Water that won’t clear no matter what chemistry you add
• Persistent foul smell that returns within days of a fresh fill
• Foam that comes back constantly
• Chlorine demand that’s much higher than it should be
• Filter that clogs unusually fast

Almost every hot tub older than a couple years has some biofilm. The question is how bad and what to do about it.

The Fix: Line Flush + Full Drain

The solution is a line flush product (sometimes called ‘purge’ or ‘system flush’) followed by a full drain, surface clean, and refill. The flush product breaks down the biofilm matrix, the jets and pumps move it through the lines, and the drain takes it all out.

Going through this once a year, at the same time as your normal water changes, prevents the cycle from getting bad again.

Other Reasons Water Stays Cloudy

Not every cloudy-water problem is biofilm. Other common causes:

• High calcium hardness. Causes scaling and cloudiness. Affects older fills more than new ones.
• Wrong pH. High pH (above 7.8) precipitates dissolved minerals and clouds water.
• Filter saturation. A filter loaded with debris can’t catch new debris.
• Bather load. Lotions, sunscreen, hair products, and sweat overload the sanitizer.
• Insufficient sanitizer. Chlorine or bromine levels dropping below the active range.
• Water source. Some Nashville-area water sources are higher in minerals or organic content.

Smart Maintenance Habits

• Rinse the filter monthly, soak quarterly, replace yearly
• Drain and refill every 3–4 months (or use a fresh water indicator)
• Shock weekly if used regularly
• Check chemistry 2–3 times per week minimum
• Shower before getting in (cuts bather load dramatically)
• Line flush at every drain and refill

What Shortcuts Backfire

• ‘Just adding more chlorine’ doesn’t address biofilm
• Skipping the line flush at refill because the water looks clean
• Cleaning the filter with the pump running
• Adding sanitizer without checking pH first
• Topping off rather than fully refilling beyond 6 months

Frequently Asked Questions

How do I know if I have biofilm?

Persistent cloudiness, foul smell, high chlorine demand, and slime in the lines that you can sometimes see when you remove the filter.

Will a line flush damage my hot tub?

A proper hot-tub-specific line flush product won’t. Don’t use bleach or random cleaners — use products designed for this.

How often should I flush the lines?

Once per year at minimum, ideally at every drain and refill. Annually in lightly used tubs; quarterly in heavily used ones.

Can I prevent biofilm entirely?

You can dramatically slow it. Daily sanitizer, weekly shock, monthly filter rinse, quarterly drain, annual line flush. That’s the prevention recipe.