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Huh??? High chlorine or nitrates (which are bad) can render the silver ineffective, so a lower chlorine level is recommended. BTW, a pool or spas recommended chlorine levels are not considered high levels. If your levels are high enough to interfere with the silver it is too high to use. You can do whatever you like. I personally have used them for a decade or so and recommend them to my customers. As a secondary sanitizer and to lower the required chlorine level.

Spent two weeks in the UK last summer (1 week in London and 1 week in Scotland in Edinburgh) and have a close friend that lives in Tiptree who moved to the UK close to 30 years ago. It's a lovely country! Now to business: IF the stains are iron a few things to know: 1 Ascorbic acid applied directly to an iron stain WILL make it disappear in less than a minute, which is why the vitamin C test is important. If you "rubbed it around for a couple of minutes" and then believed "it improved the staining" still makes me think the stains are not iron. See if you can get some plain vitamin c (ascorbic acid) and repeat the test. 2. GLDA ‐Na4 is about half as effective at chelating iron as EDTA at normal pool/spa water pH range and EDTA is LESS THAN half as effective as HEDP and other phosphonates. All three are broken down by UV light and oxidizing agents (chlorine, bromine H202, ozone, etc.) so if metals are present in the water the reapplication of sequestrant on a regular basis as well as making sure pH does NOT spike is necessary since high pH conditions are favorable for staining. Likewise, often low pH (but not lower than 7.0 since it can be damaging to equipment) can often lift stains and redissolve them in the water. pH then needs to be brought up slowly by outgassing CO2 (aeration of the water) rather than by chemical means and sequestrant added to help prevent redepositing of stains.. 3. Ascorbic acid (vitamin C), Citric acid, and Oxalic acid are all reducing agents and capable of reducing many metal stains (but most effective on iron). They do not work in the presence of oxidizers (pool/spa sanitizers and shocks) so sanitizer levels have to be very low before application. Ascorbic acid is the least toxic of the three and oxalic acid the most but all three are used in commercial pool and spa products for stain removal. Once they have removed the stain pH needs to be kept between 7.0 and 7.4 and sanitizer added in small doses until sanitizer levels are holding. This means that the reducing agent is gone. Sequestrant also needs to be added. If the oxidizer (sanitizer or shock) is added too quickly or the pH spikes then the metals can precipitate out as stain again. 4. Iron stains are normally seen as a yellow to brownish discoloration of the entire surface of the pool or spa below the water line. They are usually not seen as round stains as your picture shows. They can cause a spot stain if a piece of metal such as a screw was sitting on the bottom of the spa and started to rust. Iron stains are essentially rust deposited on the surface of the pool or spa from dissoved iron in the water that is caused to precipitate out on the surface because of water chemistry conditions (high pH and high Oxidizer levels . IF the stains are not iron then the treatment depends on the metal (manganese, calcium, copper, silver) that caused the stain. To the best of my knowledge staining from silver is impossible to remove and you are using a silver'/H2O2 based sanitizer. Silver nitrate is the most common form of "ionic" silver used in these preparations and Silver nitrate stains most surfaces and can stain skin and hair also. Staining is also a problem in systems that use ionizers (copper, silver/copper, silver/zinc) to 'sanitize' the water. (They have been found to be ineffective because of slow kill times for many pathogens to used as a primary sanitizer without bromine or chlorine/) This ability to stain (skin and hair) is why silver nitrate is the active ingredient in some lash and brow tinting products in the cosmetology trade and was historically used as a hair dye before the advent of modern oxidation dyes (which cannot safely be used on the lashes and brows). (I know I've gone off topic but I am also very familiar with much cosmetic chemistry). Finally, you gave me a clue when you said that the water turns green for a day when you add the oxidizer (peroxide) and the clears. The ONLY thing I know of that will make pool/spa water green after adding an oxidizer is dissolved COPPER and copper can plate out in round or irregular stains. Copper stains cam be black, blue, green, tan, or metallic copper color. Copper stains are difficult to remove and sometimes impossible. Best chance at removing them without calling in a professional stain removal service (not sure if you have them in the UK but we have them on this side of the pond) is to drop the pH to between 7.0 and 7.4 and maintain it like that for at least a few weeks while adding sequestrant weekly and if the stains lift drain and refill with water that does not contain copper or other metals, it is better to keep the pH at the low end of the range and do not let it rise above 7.4. Bottom line, there is no easy way to get rid of stains, no "magic potion", no "miracle in a bottle". It's detective work and trial and error. I gave you some guidelines to follow. I hope something in here is helpful and good luck.