Red Marks On Corydoras Aeneus

[Purity]

Hi, I came back from school and fed the fish in my 65 litre tank and found one of my cories covered in red marks, there are marks on his fins his head and half of his body. I immediately moved him to my quarantine tank and added an extra filter to it. I'm copying my log book out so you can help.

Tank: ViaAqua 418
Equipment:
Heater - Aqua One 55w
Filtration - Undergravel Filter (Make Unknown)
Fluval 1 Plus
Plants:
4 Cryptocorynes (Type Unknown)
Stock:
3x Corydoras Aeneus (Added 24th Nov)
2x Colisa Lalia (Added 29th Nov)

Thanks, I love all my fish with my heart and I really want to know what's wrong with him.
Joe

PS: He's alive but he's very still.

 

[WILDER]

Your filter not adequate for that tank size.
Best to over filter a tank.
You need at least a fluval 2.
Can you post your water stats in ammonia, nitrite, nitrate, and ph.
Red streaking is bad water quality to septicemia.
I would add anti internal bacteria med by interpet.
Septicemia hard to cure once it advances as it blood poisoning.

Sorry you have an undergravel filter but they cause alot of problems and are outdated now.
I would buy a fluval 2 and put the fluval one sponges into the fluval two.
Once the filter cycled I would remove the undergravel filter.