I have just two goldfish in a 39-inch long 40-gallon tank; one of them is about seven inches long, excluding the tail and is a "standard" goldfish-shape; the other fish is a bit smaller, with long tailfins. However, I have a regular recurrence of what appears to be finrot, every four or five months.

It is the larger fish that seems to be affected. He/she becomes listless for a day or so, then the front fins start to show redness at the bases and then into the fin-ray tissue, along the actual rays. If not spotted quickly, the tailfin begins to produce the same colouring, also the dorsal one. But they don't shred or go fluffy.
I have been counteracting these effects by using a proprietary anti-finrot treatment, which seems to work well....but I am actually unsure as to whether it is finrot or not; I dont allow it to develop in order to find out!

Last year, the problem was prevalent during the hot weather; I also had problems with keeping nitrite levels down and, following an Internet search for answers, had to add NutriCycle organisms to the tank since it was possible that the bacteria in the filter were struggling.
So, I'm wondering whether the fin problems are due to creeping nitrite levels or whether it is a proper finrot. I'm trying to get a grip on the problem before this year's summer holidays.

The thing is, a 7-inch goldfish is a bit big for a 39 gallon tank, a fish of this size should ideally be in a garden pond or a 5 x 2 x 2ft tank as a minimum. Overstocking with fish that are too big can result in poor water conditions, which in turn causes disease.

Cases of finrot often occur after the onset of a fungal infection. Adding bottled cycling products that claim to contain live bacteria are more or less next to useless and a complete waste of money.

Have you tested the water for pH, ammonia, nitrite and nitrate?

pH was around 6.5-7; nitrites were 0.3 but I've since done a 30% water change; havent been able to get a nitrate kit locally so have had to order one from elsewhere.
If you feel the 7" fish is too big for the tank, maybe I will have to change the water more often than I currently do?, because I havent got an outside pond (or a bigger tank) to put him/her in.....he's seven years old now.