The 72h come from the shortest excystment time, not from how long the parasite can stay on the fish. The latter is actually 7 days and the reason why you have to do 3 transfers to clean the fish up. The 4th transfer is a safety measure.
Copper, at the correct dosage, should kill all free ich stages (tomite and protomont) pretty much within minutes, but the feeding and cysts stages are shielded from it by either the fish's skin (theront) or the shell of the cysts (tomont).
The 2 main issues with copper are:
1. The minimum lethal dose for the parasite is pretty much also the highest dose many fish can handle.
2. Copper is an immunosuppressant, and as such may open the door for other infections. Lymphocystis is the most common.
As for velvet - copper may or may not work depending on the velvet strain you are dealing with. Some can handle twice the lethal dose for fish.
While Chloroquine Phosphate is generally the better weapon of choice against this parasite, it has its limits as well.
1. It can't be used with all fish species: Pipefish, Seahorses and many Wrasses can't handle it and have to be treated with copper.
2. CP is a tricky drug since there is no cheap way to test for it. Age may reduce its effectiveness. The biggest concerns are that certain bacteria may break it down and the fact that it likes to attach to glass surfaces.
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I.feel this is all true, and if so, 8 days in copper, should eradicate it. If the longest time the fish can carry it in them, protected by their skin, is 7 days, then by going 8 (call it 10 for insurance), days of copper would kill anything not on the fish.
The ones that happen to encyst at the bottom, well great, the fish is removed after 10 days to the display. If they hatch out while the fish is present, well, then so is the copper.
To further visualize (again thinking out loud) the two stages copper.kills, the free swimmers going to and from the fish, can't live with copper. The ones embedded on the fish can, but for only 7 days. Waiting that 10 days, would mean they are all of the fish by now,i n route to the bottom, but copper should stop that.
That takes care of 3/4 stages. The last, is the encysted ones. Of they hatch while the fish is in there , well now there free swimmers, and met with copper. If they hatch after the 10 days, well the fish is already gone and there is no host.
Please correct me. By going 10 days, the copper will meet everything ich wise.e except for what lays in the protected cyst stage. One of two things will happen after 10 days. The fish will be.moved, and that cyst is null and void, or it will hatch and have to overcome copper.