Using BBB method, chemistry is perfect. Water when spa is not pumping is very clear. If I run the jets, then stop them, and look at the water illuminated by the underwater light, and wait for the bubbles to disperse, there are teeny tiny little white flakes swirling around. At first I thought they were bubbles, but they're not. It looks like dust, or lint--little bits of fluff. Shocking the water with bleach doesn't make them go away. Cleaning the filter doesn't make them go away. It's not a dense cloud, but it's annoying.

Any ideas?

Sounds like small particles like dead skin, but normally such particles get caught in cartridge filters. Perhaps you've got a tear in yours or for some reason the water isn't circulating through the filter well. If there isn't very much of it, then it could be similar to what is seen in pools when the underwater light is on (see this post, but turnover rates are much longer in pools which is why such particles can stick around longer.

Indeed, it looks like the photos in that link.

Our filter never seems particularly dirty. I'll try soaking it to make sure.

The design of the spa has the jets from about halfway to the water surface, with a big volume below the level of the jets where feet, etc. go, and the skimmer is on the surface. If the jets aren't mixing the water thoroughly, including circulating the bottom water upwards, I guess stuff could accumulate on the bottom, but that would be a bad design.

If your circulation jets are directional at all, you could try pointing them in different directions. Usually, pointing slightly downward and diagonal to create a swirling motion works reasonably well (if the jets at least halfway up from the bottom to the water surface). Then you just have to deal with the surface water, but without a skimmer equivalent that might be more difficult, especially for things that tend to float. The aeration jets might cause enough surface disruption to get that water circulated and mixed downwards.

Hey thanks, I never thought of that! Currently the jets are pointed horizontally, and we don't use some of the banks of them (but I don't shut them off because there's plenty of jet pressure), so I'll point those downwards and see if it helps.

We can also increase the amount of time on filter cycle.