When doing a freshwater dip, are you using Prime? Also, dipping fish isn't a super cure for all diseases, for example whitespot or marine ich, is basically pointless, marine velvet will also increase. Remember ich, velvet etc are all increased by temperature, stress levels and a wide range of other contributing factors. Dipping your fish is a stressful process on both you and the fish. To be honest you are doing nothing wrong. The way I have done it for about 6 years is literally, fill up a black bucket with water whack a heater in it until its at 24 degrees, then I match the pH with a buffer that will basically take it to 8.4, from there I put the fish in. I monitor it for 5 minutes, signs of the fish over stressing would be curling, if the fish is laying on its side I still allow it to go until it starts to breathe heavy. Remember not everything is going to be easy on the fish, the whole point of a freshwater bath is that any saltwater parasites die, both the fish and parasite must be in the water long enough for the process to occur. A black bucket is good for that because it allows the fish to feel safe in darkness, and the parasites are visible at the bottom of the bucket. I hope any of that advice helps.
