I would do a FW dip on all fish before placing them in QT. Once in QT, treat with copper and get it up to therapeutic levels ASAP (you don't have time to ramp it up slowly with velvet.)
Freshwater Dip: Provides temporary relief for Brooklynella, Flukes, Marine Velvet disease (Amyloodinium); possibly even Ich & Uronema marinum (both unproven). Can be used to confirm the presence of Flukes.
How To Treat - Fill a bucket with RODI water, and use a heater to match the temperature to the water the fish is coming from. Aerate the water heavily for at least 30 minutes prior to doing the dip, then discontinue aeration while performing the dip. Fish aren’t overly pH sensitive for short durations like this, but you can squirt a little tank water into the dip just before the fish goes in to help bring it up.
Place the fish in the freshwater (FW) dip and observe closely. It is not unusual for them to freak out a little at first. Also, tangs are notorious for “playing dead” during a FW dip. The important thing is to watch their gills; they should be breathing heavily at all times during the dip. If breathing slows, it’s time to exit the dip. Dip the fish for no longer than 5 minutes. Multiple dips may be done, but it’s important to give your fish a day to recuperate in-between dips.
For flukes, use a dark (preferably black) bucket so you can see if tiny white worms fall out of the fish (especially out of the gills) at around the 3-4 minute mark. The worms will settle to the bottom, so you can use a flashlight to look for them there as well.
Pros - Provides temporary relief for a wide range of diseases in a chemical free environment. Can “buy you more time” until a proper treatment can be done.
Cons/Side Effects - Not a permanent “fix” for any disease, as FW dips are not potent enough to eradicate all of the parasites/worms afflicting the fish. Some fish can have an adverse reaction to a FW dip by appearing unable to maintain their equilibrium once returned to the aquarium. If this happens, hold the fish upright (using latex, nitrile or rubber gloves), and gently glide him through the water (to get saltwater flowing through the gills again). It is also a good idea to place the fish in an acclimation box until he appears “normal”.
Copper: Treats Ich (Cryptocaryon irritans), Marine Velvet disease (Amyloodinium); possibly Uronema marinum
How To Treat - First, it is important to know what kind of copper you are using. Cupramine is fully charged (ionic) copper, and has a therapeutic range of 0.35-0.5 mg/L or ppm. You would use a Seachem or Salifert copper test kit for Cupramine, as those are capable of reading copper in the low range. Coppersafe, on the other hand, is chelated copper. It has a much higher therapeutic range of 1.5-2.0 mg/L or ppm. As such, you need a “total copper test kit” such as API’s to measure Coppersafe.
Standard copper treatment lasts 30 consecutive days. The reason it takes so long is copper only targets the “free swimming stage” (the same holds true for allchemical treatments & hypo). While 7-14 days is the “norm” to reach this stage, certain strains of ich have prolonged life cycles. Indeed, even 30 days may not be long enough in some rare cases. This is why it is so important to observe after treatment ends, to ensure symptoms do not return.
Therapeutic copper levels must be maintained at all times during the 30 days, so testing often is important. If the level drops even slightly out of range, then the 30 day clock starts all over again. One reason your copper level may drop unexpectedly is if you are treating in a tank with rock and substrate; those should not be used in the presence of copper due to absorption. Conversely, if you exceed the therapeutic range you risk killing the fish.
Copper is a poison, pure and simple. It only works because most fish are able to withstand being in it longer than the parasites. Knowing this, it is wise to raise your copper level very slowly (over 3-5 days) instead of the usual 24-48 hours recommended on the labels. Doing so increases your odds of successfully treating a “copper sensitive” fish. Remove copper after 30 days by running activated carbon.
Pros - Readily available
Cons/Side Effects - Appetite suppression is a common side effect. If a fish stops eating, don’t add more copper until he resumes. If the fish is still not eating after 2-3 days, start doing water changes (lowering the copper concentration) until he eats. If this happens a second time after you resume raising the copper, you’ll know you’ve encountered a “copper sensitive” fish and an alternative treatment should be used instead. Some species of fish, such as angels, puffers, lions and mandarins are notoriously difficult to treat with copper.