I've done FW dips on several occasions and never longer for 10 minutes. What I did was put some fresh RO/DI water in a 5g bucket, dropped a heater and powerhead in and waited until the water was the same temp as my tank. When all was ready I put my purple tang in for approximately 10 minutes. What happens next is easy to explain. All fish have a slime coating and when you introduce a marine fish to freshwater the stress it induces makes their slime coat fall off and with it any bacteria or pathogens that may be attached to it. My purple tang had ich really bad and this FW dip method got rid of it. Anyhow, after the dip just place your fish back into your tank and wait and see. No need to re-acclimate it is necessary. Let me know if you have any other questions.