After keeping the same water in the hot tub over the winter, I noticed it seemed to be getting a bit cloudy.
The water surface is >95% covered with wood/aluminum, CYA was reasonable (25-30) and I had been "fairly diligent" about checking & adjusting FC level daily so I figured it was the particularly dismal cloudy light rather than anything wrong with the water. Dragged the little Intex filter pump up from the basement and ran it for a few hours -- it didn't appear to make any difference, so in my little mind that reinforced the theory that it was poor light rather than poor water.
We finally got a sunny day, so I pulled off the cover and saw a noticeable greenish tinge. Not swampy, but not clear either. Colour isn't as obvious in a wooden tub as it is in a nice light-coloured pool, but with bright sun there was no doubt. Thought about SLAM-ing just to get some experience with it, but eventually decided to drain and refill.
As the water level dropped (it's about 3-1/2 feet deep when full) I was surprised that the wood grain on the seats and bottom of the tub was not becoming clear. I had assumed the only problem was cloudy water, but when I tried brushing a big plume of white powder (presumably dead algae) came up from the bottom, and the wood grain was clearly visible under the powder. Geez that's a lot of dead algae -- guess the FC had been high enough to keep it from going swampy but there was a constant grow/kill cycle going on.
Finished draining the tub while brushing constantly, then scrubbed with diluted bleach & refilled. I'm being more careful about checking FC daily rather than "daily except when I forget", hopefully that should prevent future relapses. I couldn't believe how much dead algae formed even in my little tub -- I have a lot more sympathy now for people waiting for their filter to clean up a 20K gallon pool.
Anyways, the main point that was reinforced for me was the difference between "shock" and "SLAM" -- FC level was getting up to shock level from time to time (when I was going to be away for a few days) -- apparently all that did was turn most of the live algae into dead algae but leave enough algae for growth to continue.
The second revelation for me was that I *really* need to look harder into growing specialized algae for biofuel production. I was really impressed how enthusiastically it grew