When I bought the water testing kit the guy at the garden centre told me that for PH balance if the result was at either end of the extremes that it would be a worry. As far as the scale provided with the kit 8.0-9.0 is borderline "high". The manual even states it is bad for fish *shrug*
Your statement seems a little conflicting, unless I read it wrong?
"Fish like stable pH. 8 to 9 is fine. Trying to lower it is very dangerous for the fish." ... "Adding tap water, even with chlorine, is a good way to improve water fast since you already have an overflow." > my understanding of my tap water P.H level of 7.5 being in the mid range and replacing some/most of the current pond water (P.H 8.0-9.0) in order to lower the P.H to a more nominal level (more acceptable for the fish from what the garden centre guy told me) may be a bad thing as fish like stability? Unless you meant that adding tap water would improve water quality in another way besides P.H levels? Sorry, I'm not very clued up on pond maintenance so I may get things confused or mixed up.
Stable pH
Yes, it does sound conflicting. And that's a big problem with learning about ponds. There are few absolutes, most things are trade offs. Also there's a tremendous amount of really bad info. And of the good info it depends on the type of pond. High end Koi ponds consider things like plants to be very bad while in a water garden they would be considered very good. There are valid reasons why plants are bad and valid reasons why plants are good.
I'll try to explain in an overly simplified way just to try and give you an overview. Then you can research for details if you like.
Many experienced pond keepers add baking soda to their pond. That raises the pH into the 8.3 - 9 range. This is called pH buffering.
Optimal pH is about 7.5. So why do experienced pond keepers make their pH 8.3-9? Because the baking soda keeps the pH stable so they don't have "pH crash" or "pH swings" which can be deadly. Without pH buffering pH can swing wildly, 4.0 to 10.0.
Some experienced pond keepers do try and keep pH lower
and stable, but that takes a lot of effort. Certainly not something an inexperienced keeper can pull off. So for an inexperienced keeper lowering pH can lead to dead fish.
Google "pond pH buffering" for more info.
People who sell products like to say high pH is a problem because they sell products to lower pH. I can assure you many ponds (maybe most) are kept at pH above 8, even 9 is not uncommon. High end Koi keepers report normal growth at these higher pH levels. There is debate on whether it makes the white on Koi less white, but even that has been debatable. There seems to be no risk to fish at pH 8-9.
Adding tap water...
Water from wells would not have chlorine. So I assume your water does have chlorine. Chlorine in large amounts can kill fish. If you need to add a lot there a product that can be added to neutralize the chlorine.
Trickle water change of 2-12 GPH (gallons per hour), with chlorine and no neutralizer added is considered safe for a 3000 gal pond.
That the tap water is 7.5 will probably not lower the pH of your pond at least short term. What? It's the pH buffering. Ever add an acid to baking soda? It bubbles. The baking soda is like 9.0 pH and the acid is like 2. If you take a cup of baking soda and add a drop of acid it'll bubble and stop. A little bit of the baking soda reacted with the acid cancelling each other out. Some of the baking soda is lost in the process. The pH of the cup will still be 9.0. Add more acid, still 9.0. At some point the last little bit of baking soda left is used up and bam the pH goes from 9.0 to 2. The baking soda act as a buffer, keeping pH stable. There is something in your water keeping pH up, acting as a buffer. Adding 7.5 water only uses up some of the buffer.
However, that's not specific to your pond. The level of pH in many ponds cycle through out the day. If you measured 8.0 in the afternoon you might measure 7.0 in the early morning. Google "daily pond ph swing" for more info. A more important measurement imo is KH rather than pH. Let's say KH is the measurement of the amount of buffer in the pond. From that number you know if your pH will be stable for some time in the future. Knowing KH means you really don't need to measure pH except maybe as a confirmation.
Why adding tap water is good...
Basically given a pond that has some problem that isn't understood but poor water quality is suspected a trickle water change is the easiest and safest way for an inexperienced keeper to improve quality. Let's say it makes water stable and removes problem chemicals like ammonia, nitrites, nitrates and other problems. And it's a slow change so you don't have to learn a bunch of stuff about the dangers of fast temp and pH changes. Here's a video that explains it.
Back to dropsy...as mentioned previously that dropsy is more of a symptom than a disease I would assume that I would not need to remove the affected fish and quarantine them as it is not a contagious disease but more likely a combination of negative factors contributing to ill health?
Sounds like a reasonable guess.
As for my water supply. I'm not entirely sure what you are asking, sorry. My family home is in a suburban area, a small town. As far as I am aware most if not all of the water within the UK has some chlorine in it. I believe it is "soft" water and unlike built up city areas like London there is no chalky residue in the water. Not sure how helpful this info is.
I'd say you have that optimal water for Goldfish and Koi many people would love to have. That makes a trickle water change system a great choice for you. You wouldn't have to worry about many things like baking soda, pH, etc.