UK- Golfish Septicemia

Question
Hi there.
My four goldfish appear to have septicemia.
One has had it for a couple of months now but has not worsened, however after seeking advice from two local fish stores, I was told that this disease is a relatively fast killer.
Two of the others (Penguin the Lionhead Oranda and Mr Fishy the comet) have some very faint blood streaking in their tail fins, and the final one (Stevie the Shubunkin) has a single very thin line that most people would probably miss, I just adore my fish and so am constantly checking on them, worrying about them etc.
The first fish to develop it (Nemo the fantail) has had faint lines before but these have always cleared up after adding Melafix.
This time, however, the lines are much more prominent and have been this way for weeks, and Melafix and Aquarium Salt are not effective.
Other than the streaks, all the fish are in high spirits, they are very greedy and always have a good appetite, they are as playful as always and look otherwise very healthy.
Is there any other disease this could be? And is there anything else I could do? Medicine-wise, and so on.
The pH of the tank is 8.5, but it does vary between this and 7. I believe this is due to the addition of Melafix as this usually happens, and the salt.
The nitrate is below 10, the ammonia is 0 and the nitrite is 0.3, which is a little higher than I would like it to be at the moment, however I believe this is the upper limit of acceptable.
I have another tank with three goldfish, all of which are fine, and so I am struggling to understand such variation in health between the tanks.
Like I said, I love these guys and despite advice from my local stores to 'leave it' and just do frequent water changes. They don't seem to have an explaination as to why my fish are still alive due to the onset of the disease being so long ago now, nor do they know why the fish has not been affected in any other way, but they have said because of this, despite it being fatal in most, as it has not killed my fish over this long a period of time, it probably won't and I should just let it be and 'see how they get on'.
Obviously I don't want to just allow the disease to continue to attack my fish, and despite it not worsening, I would love to get my fish back to perfect health.
The tank with the troubled fish in is 60L, but I wish to buy a 400L tank so that all my fish can be in one tank.
However, I do not want to do the whole set  up of the new tank, only to stress the fish when I am ready to transfer them, potentially worsening their illness.
Please help?! I don't want to admit defeat as these little guys are so special to me.
This tank has been established for a year, but I have had previous set ups for almost three years.
Only three deaths of fish during that time, due to one being very poorly when she arrived and the other two whilst under the care of my mum when I was away on holiday.
I refuse to lose any more unnecessarily.
Thanks in advance
Laura- UK.
(Any Meds available in the UK please)

Answer
Laura,  With Goldies' the more water the better. The salt your using if it raising your pH like that sounds like sea salt and not aquarium salt which is just Epsom salts one teaspoon per ten gallons. The sea salt would buffer your water and raise it to a salt water pH which is 8.0 to  8.4. Extremely alkaline. If that's not it in that tank do a test .... test the water that you use (your source) then take some rocks of the bottom of your tank add them to the water and check it in about two hours. If there is a big change you are going to need new aggregate. I don't know why this happens but they use rock from so many places and some are not tank friendly after a period of time.

       If they have septicemia melafix is not a strong enough antibiotic. Treat this tough and fast. Go and get Tetracycline and add as directed. Treat the tank for at least 7 days. Do not forget you carbon needs to be removed from your filter but keep the filter running. That should take care of that problem. You need to get pH situation under control in that tank. The drastic changes are going to cause stress and other problems as you can see. Please let me know how everything goes. Good Luck, Tina