You may be right. The water is shallow, 14-16 inches deep. The sun is bright on the feature. I have tried heavy chlorine (shock) and heavy acid to no effect. I think I will try to get it balanced and try the Cyanuric acid.
Go to a pool supply store and get a test kit. With it you can make simple tests (low tech) of PH (acid/base) target should be close to 7, it also gives readings of the chlorine concentration. The typical test kit consists of a plastic block with two tubes molded in to hold your water sample. There should be a varying color scale next to each tube. The kit should include two small (like eye drops) reagent bottles w/ built in droppers, the contents of the red capped bottle are used to test acid concentration, the yellow is used to measure Cl content.
There are instructions included in the kit but the procedure it describes is very simple. The idea is to secure a water sample near the bottom of the water empoundment. Hold the test block in one hand with two fingers over the fill holes. Reach toward the bottom, uncover the holes and allow water to fill both tubes. Recap the holes w/ your fingers before bringing the test block to the surface. Uncover one of the tubes and tip the block to allow a few drops to drain then check if the water level in the tube is down to the "fill" line. Repeat with the other tube. If you are below the fill line, repeat the above procedure until you have both tubes filled with the correct volume of water. The instructions will tell you to uncap the reagent bottles and add PP drops from the red capped bottle to the PH tube, repeat w/ CC drops from the yellow capped bottle on the Cl tube. Re-cover the tops of the tubes with your fingers and shake the block to mix the reagents /w the water sample. To read the results, turn so that you are not looking at the sun nor is it behind your back. Hold the block up so that you are looking at the sky. Note the number on the color scale that is the closest match to the color of the sample.
Over time you will get a feel for how much Cl to use and also try to hold the Ph near 7. Too much acid and you get corrosion on pumps and related plumbing (metal parts). Cl does not evaporate as such, it is consumed when it attacks the organic debris that accumulates in your water, direct sunlight also increases the Cl loss.
Adding cyanuric acid keeps the Cl levels more stable and most pool chemical supply outlets sell it as "stabilizer". There are test kits for "cyanuric acid" level but they are expensive and are not really necessary. The usual dosage is listed as pounds per gallon. Unless you have to add a lot of water over time the cyanuric acid concentration should stay nearly constant over a full season.
Cyanuric acid acts like "sun blocker" for the chlorine and eliminates the need to shock to maintain Cl levels.
Regards & Best of Luck,
GtG