Lethargic Oranda

Size of aquarium/pond - 170L Approx
Types of fish - 1x Oranda,1x Blackmoor
Number of fish and sizes - small blackmoor, medium oranda
Filtration used and how do you clean it and how often - Fluval 206, Fluval U3 cleaned every other month in terns
How much in the way of water changes and how often -- 50L waterchange weekly
Do you dechlorinate - i do
Symptoms of sick fish ie, spots, behaviour, etc - oranda seems less energetic, dorsal fin is almost flat to back looks to have white spots forming on tail
How long has the fish been sick and which medications are your currently using or recently used - 4-5 days
Test results for ammonia, nitrite, nitrate and ph both from the tap and tank - PH= 7.6 Ammonia 0.50PPM, Nitrite 0PPM Nitrate 20PPM
Anything else you need us to know -
 
so have just tested and have added prime into the water for the ammonia, i got the blackmoor excatly 3 weeks ago from the indipendant fish store i get every thing fish related from, at the same time i changed the carbon in the 206 filter for zeo-carb. i have a divider net so i can split the tank to keep them apart if i need to. please help, i love that fish more then the dog

what is the dimensions of the tank? i'm not very good at working in ltrs
 
what kina size are the white spots and do they look clearly defined  or kina fuzzy round the edges?

only very small but quite clearly defined, more noticable on the tail then anywhere else, and i havent got a tape measure sorry

more noticable on the tail then anywhere else. does this mean they are elsewhere other than it's tail?
 
sounds kina like white spot/ ich.
i've never had to deal with this but i know the parasite can only be killed in the free swimming stage, unfortunately that's all i know.
seen as you haven't had much of a response here (as most on this site are tropical keepers) try posting under the title ich in the newbie section, you will prob get a better response and someone will be able to tell you how to go about treating it.