colour of faeces

I have a discus PB which acts normal eats normal but having a white faeces with bits of black in it , i always was under the impression that if the faeces were white ,transparent,or like cotton wool then the fish has got internal infection or worms and the drug of choise is metro but i don't know what is this.
has any body else experience this and what is medication used.
the tank is 500 liter,5 adult discus,3 b/n cat fish,6 coridoras, 4 ruminose and 8 harlequin rasbora,amm 0,nitrite 0 ,nitrate 10 ,ph 6.6,gh 4 ,kh 2, temp 30 c
also the other discus are fine.your thoghts and advice are appricieted.
cheers
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update
i put the fish in a hospital tank BB 40 liter and treated with metro 400 mg ,one tablet per day for 10 days 33 c temperature, the colour of poo changed to dark after 2 days .
after 10 days transferred to main tank within a week the white poo has come back ,every other fish is o.k i have just finished a course of Kusuri in the main tank but still it has got the white poo,some time it is half brown half white ,he acts normal in every other way and eats and plays like nothing is wrong .http://i1378.photobucket.com/albums/...ps4fptyx6i.jpg
your thoughts and comments are appreciated.