My hot tub was fine until two weeks ago. Water clean, no smell, almost no buildup. Piping's clean, too. It's all the same today. Only now there's an unpleasant, rough coating on the tub. Mostly under the edge of the water. But generally in the entire pool. What kind of coating is that? And what can I do to get it off?
This rough surface is a lime deposit. You've had a lime deposit. To remove the rough coating, lower the pH of the water to 5.0 using an electronic pH tester or DPD tablets. Under no circumstances use test strips. The test strips are too inaccurate when reading the pH value. When you have lowered the pH value, the rough coating should disappear on its own within two days. Keep the pH value at 5.0 until the coating is gone.
Under no circumstances should you remove the rough coating mechanically. This coating is also in the pipelines and would lead to biofilms if you do not remove it there. The rough coating consists of lime. By lowering the pH-value of the water, it becomes an "acid" and thus dissolves the basic lime. A pH-value of 5.0 is not corrosive to the technique for this short time. You can also use the whirlpool during this time.
The skin has a pH of 5.5 and tolerates this water well. When the limescale has disappeared, raise the pH value back to 6.5 to 6.8. If the pH value is below 7.0, no limescale deposits can form. At 6.5 to 6.8 you do not need to worry about corrosion. Since the Lotus Clean products, unlike chlorine, do not introduce any acid into the water, the water is not corrosive to the technology at these pH values.
You can find further explanations about pH values and lime here: