help sick chichlid!

Question
ive had my tank for a year and half. i was given a turtle and cichlid as a
present from a friend who was moving. they have lived very well togther for at
least 2 years before i got them. my cichlid is at least 7 in long and has been
very active and colorful in the past. i have one small filter in the tank and
one heat lamp. i think the tank is at least 90 gallons. (it took a long time to
fill) i treat the water only with declorinator and water cleaner to break down
turtle waste and up unitl recently have had no issues. ive noticed that the my
cichlid has been acting odd, he sits in the corner of the tank, or at the
bottom, and doesnt move often, sometimes he will eat. today i noticed an open
sore on his nose between his eyes, its not very deep but it has a white around
the wound. what do i do? i dont want to start treating the tank for bacteria or
infection and hurt the turtle (hes a small southern snapper) what do i do?
please help!

Answer
It could either be an infected wound or something worse, Hexamita. Hexamita is treatable. An effective treatment is the drug metronidazole. A combined treatment in the food (1% in any food the fish will eat) and in the water (12 mg per liter) is recommended. Repeat the water treatment every other day for three treatments.
Antibiotics will treat the fish if it's a simple infection.
I would put the turtle in a seperate tank while treating the fish