So, let me get this straight. You have six goldfish that are seven years old each? As well as some others? Okay, well as you know, the older fish get, the weaker their immune systems get. Do just the older fish have the redness on their fins?
Fin redness can most times be a sign of fungus. I would use a remedy called pimafix, which makes use of pimenta racemosa - or extract from West Indian bay trees. It's a 7-day anti-fungal treatment, where you add a little more to the water each day. I have cured 3 fish with this remedy, and highly reccomend it.
If you do choose to use pimafix however, be sure to remove any carbon from your filter.