You have 2 options:
- (Preferred) QT all your fish (inverts remain in the DT) and treat for ich using one of the options mentioned here: https://www.reef2reef.com/threads/how-to-treat-ich.191226/. Copper might be your best bet just in case this is velvet, but raise it slowly (4-5 days) because of the angel. I also wouldn't QT that bio-load in anything less than a 10 gal QT.
- (Risky) Remove all your inverts and treat with hyposalinity in the DT (see below). This only treats ich (not velvet) and has had mixed results when done in a DT. But I still think I'd rather see you do this than put 3 fish in a 5 gal QT.
Hyposalinity: Treats Ich (Cryptocaryon irritans)
only.
How To Treat - Place the fish you wish to treat in a quarantine tank with SG & temperature matching the tank they came from. Over a period of 48 hours, gradually lower the SG down to 1.009. You must use a
perfectly calibrated refractometer at all times while doing hypo. Treat for 30 consecutive days, and during that time the SG must always remain at 1.009. If it inches up even slightly, the 30 day clock restarts. For this reason, many people use an auto top off system while performing hyposalinity. Some have even used hypo to successfully rid their display tank of ich, while others have failed. All corals and inverts must be removed beforehand if you wish to try this.
One of the challenges posed by hypo is maintaining a proper pH for the entire duration. While fish aren’t overly sensitive to low pH for short periods of time, anything continuously lower than 7.5 is going to be a problem. So, you will have to constantly test and then buffer the water to raise the pH. This can be accomplished by using supplements (available at most LFS) or you can “bake” your own DIY supplement by using baking soda. Spread baking soda onto a clean baking sheet, and bake at 300F for 1 hour. This process drives off water and carbon dioxide from the baking soda, and the result is an effective pH buffer. You will need to experiment (start with a very small amount) to determine how much is needed to raise your pH to the desired level.
Pros - Chemical free solution to ich, gentle on the fish.
Cons/Side Effects - Difficult to execute properly, and hypo resistant strains of ich have been proven to exist (study done by Yambot in 2003.)