What Causes Pitting in an Aluminum Hot Tub?
Two different things get called pitting, and they have different causes. Worth separating them before doing anything about them.
Surface deposits
Small hard nubs, usually white or grey, often appearing within days of a fill. These are mineral scale or aluminum salts precipitating out of the water onto the surface. They scrape off with a plastic edge and they don't reduce the thickness of the metal.
Dulling
The surface goes from bright to matte. This is oxidation, which is the metal protecting itself. Not damage, and it stabilises.
The cause most owners don't expect is water that's too soft
Water with very low calcium hardness is chemically aggressive. It's under saturated, and it pulls minerals from whatever it touches, including the structure of the tub.
Some municipal supplies run as low as 10 ppm calcium. A water softener strips calcium by design, so a tub filled through one is below spec before anything else has happened. Our target range is 150 to 250 ppm, and it's worth testing on every fill and weekly after filling, rather than assuming.
Anodes and grounding are the two things people usually try first when deposits appear. Neither addresses this particular mechanism. If the water itself is aggressive, the fix is calcium.
Dissimilar metals
Copper, brass and some stainless components in contact with aluminum in water set up a galvanic cell, and aluminum is the one that gives way.
This is concentrated rather than general. It shows up at the point where two metals meet, which is why deposits clustered around a fitting point to the fitting rather than to the water. An anode elsewhere in the tub doesn't reach a cell at a fitting.
Any aftermarket hardware is worth checking before it goes anywhere near the tub. If you're unsure about a part, ask us first.
Incompatible water treatments
Chlorine in any form, salt chlorine generators, copper based algaecides and silver based mineral cartridges are not compatible with an aluminum interior. Several of these are standard for acrylic spas, which is why they get recommended by pool stores that haven't seen an aluminum tub before.
Bromine at 2 to 3 ppm, hydrogen peroxide and plant derived treatments are what we specify. If you'd rather not work out product by product what qualifies, Good Clean Living Water Care is a curated collection built specifically around aluminum and around our published ranges, so everything in it is compatible by design.
Anodes past their interval
A consumed anode provides no protection. Replace every 3 to 4 months, or once around 30% remains.
What this means in practice
Surface changes in aluminum come down to a small number of identifiable causes, and all of them are addressable. That's why we publish specific numbers rather than general advice.
Every AlumiTub built since June 2026 is also built with a marine grade protective barrier at the aluminum surface, so the tub carries that protection itself rather than relying on the guidance alone. We still publish water care ranges and still ship anodes, because balanced water protects the equipment, the cedar and the quality of your soak, not only the structure. The difference is that the tub is now built to handle a mistake.
Restoring a tub that already has deposits
Deposits are generally removable and generally cosmetic. Beyond that, the protective barrier can be applied to tubs already in service.
Email hi@alumitubs.com with photos and your water numbers. In most cases this is straightforward to resolve, and we'll walk you through it. For tubs built before June 2026, we can also apply the protective barrier so it doesn't recur.