^^ Agree
Dont use rally on fallow tank, it won't work on the cysts, also it will change the color messing up the photosynthetic light available, I dont believe it's reef safe if used more than 2 weeks because the color change will stress and mess up the corals. I did use it before and then SPS started bleaching after 2 weeks, so I removed it, but it works great in providing relief from velvet (although not cure unfortunately) so definitely do it as a bath on fish.
When doing a bath I recommend to provide plenty of aeration, as it will deplete oxygen. You can skip rally if the fish is not heavily infested with velvet, but always make sure to monitor them closely, sometimes it might be heavily infested but shows no signs except for rapid breathing (especially if it has thick coat like wrasse or clown...), if thats the case then do a bath. Velvet will suffocate the fish by infecting the gills and and slowly blocking it until the fish suffocates. But since they already survived the outbreak then they should be fine for another few days until the next life cycle, hopefully by then they will be in copper.
For the half dosage, halving the dosage mentioned on the bottle should be fine, but make sure to use the test kit for the final full dosage, as some variation might occur especially that you are using sand which might absorb some.
Also, I suggest to remove the decoration if you don't know from what it is made from, PVC will do fine for providing hiding spots. Decoration might absorb copper, leach chemicals during treatment, ...
Best of luck, and keep us updated
