Hi loveie and welcome. Sorry to hear you are having problems. Perhaps our member Vinson massif can help you as he too, lives in Hokitika (just so you know you aren't alone over there and there are other fish keepers )
Goldfish grow large and are very messy fish that produce a lot of waste. 100L is not big enough for 3 comets, let alone the new addition. Of course, they might be all right at the moment as I don't know how small they are.
Sounds like the oranda may have had problems and has spread it to the other fish. Can you take photos of them? It is always easier to tell what is happening from photographs.
White spot is a disease caused by stress. Have a look here to learn about it.
The red streaks could be a result of earlier ammonia poisoning or finrot. Look at the description here and see if that fits.
In the long run, I prefer to see comets in ponds. They are a streamlined fish meant for fast swimming and they can't do this to their full potential in a tank. Have you thought of building a pond? 8)
Is the fish really blind or do you just presume so as it is is going in circles?