It could also be that your Fish Mix expired if a friend gave it to you, they only last 2 years unopened, and 6 months after opening. And that is in cool dark area, heat and light makes it go bad faster. Check the production date on the barcode, and search for ''Biobizz expiration date'' to find out exactly when it was produced, it takes a bit of deciphering.
I had problems with Fish Mix, that thing is too powerful and i guess it only requires a little bit even on big healthy plants. I switched to BioGrow and it got better.