how do i use malachite green?

Desk is completely full... I don't think its the guppies though as they do not go near him... Though it did get bad very fast and he's overly gentle ... I can rehome the guppies or put them somewhere else. He deserves to not have his tail eaten. I have 6 on his side and three on the other and kaito who is on the other side of the divider's fins are perfectly fine
 
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