I don't know about this product, but I wouldn't use anything for parasite treatment that you cannot measure the level of concentration for. The medicine may be making him sicker. I have heard that malachite green is rather harsh.
I would switch to a hyposalinity treatment if you are sure its ich. It is very easy for the fish to tolerate and quite effective for ich.
If you must use a med, try cupramine. its a copper compound which is also tolerated well, but extremely effective.
Here is an article on hypo. Have used it myself with good results.