All absolutely true. Thanks for the correction. It has been so long since I used Aquarisol I didn't even realize it had copper in it. (This is the clear liquid sold in a clear plastic eye-drop type package, right?) Seems to be no longer available in my area, and a lot of meds that were formerly available are getting hard to find. Petco, because they are based in California where some of these chemicals are banned, no longer sells some of the most basic medications at any of its stores owing to the logistical problems and legal hassles that could ensue. Incidentally, the warnings on this product mention toxicity to invertebrates. I never much paid attention, since unwanted snails were the only invertebrates that were ever in my FW tanks, but since so many people are keeping various shrimp, now, take care to protect them.
Some Marine fish are so extremely sensitive to copper (e.g. dwarf angels) and perhaps it is owing to this and my coming at this from a SW versus FW perspective that I am particular about how copper is used. Indeed, earlier copper formulations were so toxic to some dwarf angels that some people didn't even try to treat them. It was like administering a lethal injection.
And there are instances where there isn't a lot besides copper that is effective. Marine velvet disease is one example. A number of years ago I remember a rather lively, fiery discussion took place in the pages of Tropical Fish Hobbyist magazine surrounding whether or not copper should ever be used. Many marine keepers were complaining that long-term copper exposure produced negative long-term effects on fish (shortened life-span, organ damage) but to my knowledge no comprehensive data was collated to make the case one way or the other. So I, along with lots of other SW folk, sort of lived with the idea of using copper as "the only effective but nevertheless really dangerous and potentially deadly medication out there." My complaint to people who spoke against the use of copper was that they never gave a very good alternative treatment for Amyloodinium which is 100% fatal to fish, whereas copper, though arguably damaging, but can also be lifesaving. I rely on copper frequently, but I think doubly hard about what I am doing when dosing it, doing water changes and replacing a partial dose, etcetera.
With medications like Quick cure and Methylene blue, you get pretty specific dosing instructions: drops per gallon etcetera, and the emptier the tank, the easier it is to be accurate with your dosing. With many copper meds, such as Coppersafe, the instructions actually tell you to measure the copper with a copper test kit to determine whether you have achieved the critical level of the active ingredient. CopperPower which I just recently used with very good results on some new fish, is a much safer product, and instructs you use it at a rate of 1/2 oz per ten gallons. But each of these medications employs a different chemical form of copper, and not all copper is created equally: some forms of it are safer than others, some are longer lasting and more stable. By the way, both medications mentioned in this paragraph are intended for use in SW, so there's that, too.
I would still advise using a hospital tank even if using Methylene or QuickCure, since the staining potential is so real for these meds. I have experienced blue gravel, blue aquarium sealant, blue rockwork, and, as an additional warning, a spotty blue bathtub bottom from where a bucket of blue water met up with some scrubbing sponges my cats happened to knock into the tub. The resulting rectangular marks are a lasting tribute to the aesthetic risks of Methylene blue! Can you imagine what the stuff would do to white carpeting!?