The only prophylactic treatment I do is a Formalin dip to prevent Brooklynella, Trichodina or Uronema from making it into the QT..
Otherwise I only treat when it is needed.
Too many people here marinate their new fish in way too many chemicals without an actual need for it.
Even prophylactic TTM treatment of new arrivals can be counter productive as it only addresses the least concerning parasite (Cryptocaryon) and may make you miss more serious infections. IMO TTM is best done towards the end of the quarantine period before the fish go into the DT or if the fish actually come down with an ich infection. But I found hyposalinity to be an easier and at least equally reliable treatment against ich.
The purpose of quarantine is to observe and see if new fish may have parasites or an infection that could be harmful to the established fish in the DT. Any prophylactic treatment can obscure or delay the outbreak of an occult infection and make you miss it.
I personally had a good deal of fish die of infections in quarantine, mostly of some serious and seemingly antibiotic resistant bacterial infections, or just general exhaustion from capture and shipping. In many cases blind treatments can make things worse. Ideally you should only treat the fish after you know what you are dealing with and how to treat it most effectively.
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