I suggest you keep doing water changes and also take some water from the well keep it seperate check ph and then let it stand a week then do another ph test if only for elimination purposes. All your fish with the exception of the harlequins (6.5) come from ph7 and above.
Finrot occurs in fish that are in poor condition usually due to rough handling,fighting,fin nipping,overcrowding,incorrect water,poor feeding and very low temperatures(below 10C) in the case of fancy coldwater fish with trailing fins.
If your tap water is 7.5 regular water changes should keep it fairly close to that, the high ph is probably killing your plants. Sewage and agricultural run-off would cause ph to rise,how does your water get to the tap? Or maybe something in the tank forcing the ph up, e.g. decorations? Also what fish are you keeping in this tank?
I visited a friend who has been having trouble with fancy goldfish dieing.Checked ph in tank 1 (6.3) tank 2 ...............wait for it a staggering 4.5! Yes 4.5 did test 3 times. After picking myself off the floor checked hospital tank and believe it or not it was over 8. That was my morning.
If it is gas bubble disease it is caused by the water being supersaturated with oxygen (water holds 140% oxygen it normally holds at that temp.) Normally associated with algal bloom and 'rich' water.
I know someone who just lost her " globe eye fantail" as she calls it,same fat belly as dropsy but without the scales sticking out. Also the eyes were bulging and dark. Any suggestions ?
Does anyone know who the seller is? Cos we do not need this sort of situation in NZ.Cos it is hard enough to get the fish we really crave under normal circumstances.