Should new arrivals be quarantined?

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I started out quarantining everything. The last 3 or 4 fish I didn't quarantine. I am taking a risk by not quarantining. I only buy fish from a reputable dealer (J and L) and I know they use copper in their systems and have good livestock-handling procedures.

One tip is it is almost impossible to quarantine a tang, I found my tangs did miserably in a small quarantine tank, they quickly pollute the water, unless you have a huge quarantine tank they do much better in the display tank.

Anyway I figure quarantining is the optimal solution but to do it properly (4 to 6 weeks, large quarantine tank with good water quality) is difficult.