Thank you AW7020, this gives good comfort. I don't think I have problems anymore with the anchor worms, you see, they were unable to adapt to % changes in salt content of the water and its salinity.
Anyway, signs of infection has now reduced, swims a lot better than before and almost back to normal. One thing I did find when I got home this morning is that the water has turned red. I believe it was because of the gram-negative bacteria and exposure of tetracycline to sunlight that caused this (foam appeared the morning before I left), at least it indicates my medication work (even if the one I used is for humans - we don't have tetracycline for fish here).
Anyway, does someone know what to do when the water has turned red? Should I replace the water and bring it back to normal before adding another dose or should I leave it alone, do the WC and add again the dose disregarding what color the water is after the water change?
I have read somewhere that this red water is toxic to us, not sure about the fish though... thanks in advance for any advice.ü