FWIW, I have used this product before, albeit in a bare QT tank. It was effective on ich.
While the bottle I have does say "Reef Safe", it's not something I'd consider using as in-tank treatment for a reef with any stoney corals.
<3% formaldehyde, but the formalin they're using is 37% formaldehyde:
Contains: A proprietary formula of water and formaldehyde (< 3%).
Purpose and Benefits
Aquarium Solutions Ich-X SW is a convenient, pre-diluted, formulation of formalin (37% formaldehyde solution) which is universally recognized as an effective medication for the treatment and control of diseases in saltwater aquarium fishes caused by fungi, protozoa and certain metazoans. When used as directed, Aquarium Solutions Ich-X SW will control or prevent diseases of fishes caused by the following disease organisms: Cryptocaryon (saltwater "Ich"), Amyloodinium, the common external fungal infections, certain external bacterial infections and diseases caused by monogenea and digenea.
Meaning it's in the neighborhood of 8% formalin.