For those who prophylactically treat with copper for ich & velvet in QT, it's an age old question: Do I treat with copper immediately or wait until the fish begins eating? Both options have solid arguments in their favor. Copper can suppress appetite, so it's a good idea to let a finicky eater begin feeding before starting treatment. However, if the fish came to you with parasites (especially velvet) then delaying treatment could mean the difference between life & death. 
In an effort to provide some clarity to this quagmire, I re-examined the "front end" of both ich & velvet's known lifecycle. This front end (after trophonts drop off the fish) is a lot more predictable than the back end (waiting for free swimmers to be released) of the lifecycle. For the sake of argument, we will assume worst case scenario for both ich & velvet. Meaning, we assume trophonts start falling off an infected specimen immediately after being placed in QT.
ICH: Once a trophont leaves the fish, it becomes a protomont. This phase crawls for anywhere from 2 to 18 hours before sticking to a surface & encysting. The cyst fully hardens in about 8-12 hours, and is now considered a tomont. This is the "egg stage" which releases theronts (free swimmers which infect fish). The time required for theront development varies/is unpredictable. However, the soonest theronts have ever emerged from a group of tomonts was 3 days or 72 hours. You'll notice this lines up perfectly with when tank transfer method is supposed to be done.
VELVET: Velvet has a lifecycle similar to ich, except velvet free swimmers are called dinospores instead of theronts. And it only takes 2 days or 48 hours before dinospores can begin emerging from velvet tomonts.
So, why is the free swimming stage so important? Because it's the only stage proven to be vulnerable to copper, Chloroquine and hyposalinity treatments. Copper provides no immediate relief for the parasites on your fish, nor is it capable of penetrating through the tomont's cyst. So, it's a waiting game ... How long until the parasite reaches the free swimming stage that the copper can zap?
We now have our answers: 48 hours for velvet & 72 hours for ich.
This means you would need to get your copper level up to therapeutic* within these time frames if you didn't want any chance of your fish being reinfected. Due to the severity of velvet, I would say raising copper to therapeutic within 48 hours is necessary if you knew your specimen was infected with that disease. However, being ich is a "milder parasite" you must weigh the risk of raising copper to therapeutic within 72 hours vs. if you think your fish can handle that. Typically, I recommend taking 5-7 days to raise copper on a fish if no signs of disease are present.
So why am I treating with copper for 30 days then? Because the above information only discusses when free swimmers can first start to emerge from tomonts. With velvet, trophonts can remain on the fish for up to 4 days, and it can take up to another 20 days for all the dinospores (free swimmers) to be released from their tomonts after that. (Dinospores can use photosynthesis for energy if copper is not present to kill them. They can remain infective for up to 15 days this way, and these combined numbers are why I recommend 6 weeks fallow in a DT for velvet.) For ich, 7 days is the maximum trophonts can stay attached to a fish and 2-3 weeks is the "normal maximum" for theront release. However, in one study it did take 35 days and in another 72 days for all the theronts to be released from a group of tomonts. The presence of copper is the only thing "shielding" your fish from reinfection. Knowing these rare strains do exist, observing post treatment to ensure symptoms do not return is very important.
* Therapeutic ionic copper (ex. Cupramine) is 0.35-0.5 mg/L or ppm. Use a Seachem or Salifert copper test kit to measure.
Therapeutic chelated copper (ex. Coppersafe) is 1.5-2.0 mg/L or ppm. Use an API copper test kit to measure.
In an effort to provide some clarity to this quagmire, I re-examined the "front end" of both ich & velvet's known lifecycle. This front end (after trophonts drop off the fish) is a lot more predictable than the back end (waiting for free swimmers to be released) of the lifecycle. For the sake of argument, we will assume worst case scenario for both ich & velvet. Meaning, we assume trophonts start falling off an infected specimen immediately after being placed in QT.
ICH: Once a trophont leaves the fish, it becomes a protomont. This phase crawls for anywhere from 2 to 18 hours before sticking to a surface & encysting. The cyst fully hardens in about 8-12 hours, and is now considered a tomont. This is the "egg stage" which releases theronts (free swimmers which infect fish). The time required for theront development varies/is unpredictable. However, the soonest theronts have ever emerged from a group of tomonts was 3 days or 72 hours. You'll notice this lines up perfectly with when tank transfer method is supposed to be done.
VELVET: Velvet has a lifecycle similar to ich, except velvet free swimmers are called dinospores instead of theronts. And it only takes 2 days or 48 hours before dinospores can begin emerging from velvet tomonts.
So, why is the free swimming stage so important? Because it's the only stage proven to be vulnerable to copper, Chloroquine and hyposalinity treatments. Copper provides no immediate relief for the parasites on your fish, nor is it capable of penetrating through the tomont's cyst. So, it's a waiting game ... How long until the parasite reaches the free swimming stage that the copper can zap?
We now have our answers: 48 hours for velvet & 72 hours for ich.
This means you would need to get your copper level up to therapeutic* within these time frames if you didn't want any chance of your fish being reinfected. Due to the severity of velvet, I would say raising copper to therapeutic within 48 hours is necessary if you knew your specimen was infected with that disease. However, being ich is a "milder parasite" you must weigh the risk of raising copper to therapeutic within 72 hours vs. if you think your fish can handle that. Typically, I recommend taking 5-7 days to raise copper on a fish if no signs of disease are present.
So why am I treating with copper for 30 days then? Because the above information only discusses when free swimmers can first start to emerge from tomonts. With velvet, trophonts can remain on the fish for up to 4 days, and it can take up to another 20 days for all the dinospores (free swimmers) to be released from their tomonts after that.
(Dinospores can use photosynthesis for energy if copper is not present to kill them. They can remain infective for up to 15 days this way, and these combined numbers are why I recommend 6 weeks fallow in a DT for velvet.) For ich, 7 days is the maximum trophonts can stay attached to a fish and 2-3 weeks is the "normal maximum" for theront release. However, in one study it did take 35 days and in another 72 days for all the theronts to be released from a group of tomonts. The presence of copper is the only thing "shielding" your fish from reinfection. Knowing these rare strains do exist, observing post treatment to ensure symptoms do not return is very important.* Therapeutic ionic copper (ex. Cupramine) is 0.35-0.5 mg/L or ppm. Use a Seachem or Salifert copper test kit to measure.
Therapeutic chelated copper (ex. Coppersafe) is 1.5-2.0 mg/L or ppm. Use an API copper test kit to measure.
