I would recommend you do not do FW dips for marine velvet. Marine Velvet (Amlyoodinium) imbeds itself pretty deeply in the tissue so FW dip are not all that effective on it. I would treat with ionic copper which requires that you get a copper test kit (FasTest works well) and of course treat in a QT tank. Formalin will also kill velvet but is very harse and can easily kill your fish as well. Seacure from Aquarium Systems is ionic copper or we can mix up a batch and provide you with an excel spreadsheet for calculating the exact dosage for $10. Some fish are more sensitive to copper so let me know what type of fish you have if you choose to use copper. Ionic copper will not ruin the 55 for future use as a reef tank.
