Hot Tub Water, Honestly

Ask ten hot tub owners what their filter does, and most will say it keeps the water clean. It does — but only half the job people imagine. There is an entire category of contamination a paper cartridge will never remove, no matter how often you rinse it, and misunderstanding that one fact is the most common reason hot tub water quietly goes wrong. Here is the straight version.

01 / Filtration

Your filter can't touch the thing most people worry about

A pleated cartridge is a mechanical strainer. Hot tub cartridges are typically rated somewhere around 25–40 micron — fine enough to catch grit, hair, dead skin, insects and body oils once they clump together. All of that is suspended matter: discrete particles still floating in the water.

Total Dissolved Solids are a different beast entirely. They are dissolved — gone into solution as individual ions and molecules: calcium, magnesium, sodium, chlorides, sulphates and the residue of every chemical you've ever added. Those sit at roughly 0.1–1 nanometre. A 25-micron cartridge is working at 25,000 nanometres. The thing you want to remove is tens of thousands of times smaller than the holes meant to catch it.

So, the cartridge handles suspended solids. Dissolved solids pass straight through, every single pass, forever. Only two things remove them: a full drain-and-refresh, or reverse osmosis. No finer pleated media bridges that gap.

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02 / What TDS Is

It's a fuel gauge, not a hygiene test

This is the misconception that causes most of the confusion: people treat TDS as a measure of how dirty the water is. It isn't. TDS tells you the bulk quantity of dissolved material — and nothing about bacteria, sanitation or how safe the water is to use. You can have crystal-clear, perfectly sanitised water at 1,500 ppm and genuinely unpleasant water at 400 ppm.

Think of it as a fuel gauge for the water's working life, not a cleanliness score. It only ever rises, and when it gets high enough the water becomes harder to manage. That's the whole story — no more, no less.

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03 / The 1,500 Rule

Where the industry number really came from

The familiar advice — refresh around 1,500 ppm — was never a bathing-safety figure. It's an operational trigger. The comparison people reach for ("I wouldn't bath in water that concentrated") doesn't hold, for two reasons. First, bath and shower water is single-pass: fresh in, straight down the drain, so it never accumulates anything. Second, your tap water already carries 50–500 ppm before anyone gets in — more in hard-water areas — and nobody thinks twice about washing in it.

What 1,500 ppm actually flags is the point where the water stops behaving: sanitiser has to fight harder for the same result, pH and alkalinity turn sluggish and drift, scale and haze creep in, and you end up spending more on chemistry than a refill would cost. It's the "more trouble than it's worth" line — and it's better expressed as a figure above your fill water, not an absolute.

You cannot balance your way to lower TDS. Every dose you add to fix the water adds to the very number you're trying to control.

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04 / The Skin Question

Does high TDS harm your skin? The honest answer

No — not directly, and we won't pretend otherwise. TDS is an aggregate number; what matters for your skin is composition, not the headline figure. The clearest proof: people pay to bathe in mineral springs and Dead Sea water at TDS levels orders of magnitude above anything a domestic tub will ever reach, and that's sold as good for the skin.

The things that genuinely affect skin, eyes and comfort in a hot tub are chloramines (the harsh by-product of sanitiser meeting sweat and oils), pH imbalance, and microbial load — none of which TDS measures. TDS matters only indirectly: a high dissolved load makes your sanitiser less efficient, which makes those real factors harder to keep in check. So fresher water is worth having — but for honest reasons (reliable sanitation, comfort, clarity), not because dissolved solids are attacking your skin.

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05 / Two Different Jobs

Balancing maintains. Refreshing resets.

Chemical balancing — sanitiser, pH, alkalinity, calcium — is essential, and it's what keeps the water safe and pleasant between refreshes. But it is not a substitute for fresh water, and it won't lower TDS. Quite the opposite: every product you add dissolves into the water and pushes TDS up. Chasing "perfect" water by over-dosing simply brings the time to refresh your water forward.

The discipline that helps is accurate balancing: test properly, dose correctly, don't overshoot. Done well, that gives you good water and slows the climb. Balancing manages a fill cycle; refreshing starts a new one. You need both.

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06 / The SUPA Standard

How to know — not guess — when to refresh

Here's the part you can act on. Buy a TDS meter; they cost a around £10.00. When you refill with fresh water, take a reading — that's your baseline. Then refresh when the water climbs to the SUPA standard above it.

If you'd rather not measure, use the recognised usage formula as a backstop — a calendar estimate based on size and bather load:

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07 / The Design Advantage

Why we can recommend a fresher standard without more hassle

A tighter refresh target would normally mean more frequent draining — more water, more chemical, more effort. When you use a SUPA Heater to heat the water, it largely doesn't, and the reason is evaporation.

A major driver of rising TDS is evaporation concentration: pure water leaves as vapour, the dissolved solids stay behind, and you top up with more mineral-bearing tap water — so the level ratchets up every cycle. A conventional tub held at temperature 24/7 evaporates constantly and concentrates TDS quickly. SUPA Heated spas with its heat-on-demand design heats only when you're using it and sits cold and covered the rest of the time, so it loses far less to evaporation.

The result: a +500 standard is reached less often than a +1,500 standard on an always-hot tub. You get fresher average water, the same or fewer drain-downs, and less water and chemical used along the way. It's a running-cost and water-quality advantage that falls straight out of the engineering — not a slogan.

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This article is general guidance on hot tub water care and is not a substitute for your product manual or professional water-testing advice. Recommended TDS targets are operating guidance for water quality and equipment longevity, not health or medical claims.

SUPA Heater, SUPA Spa and SUPA Hot Tubs · World's first heat-on-demand hot tubs. Engineered to heat fast, run only when you physically use them and keep your water working for longer.