Here are some tank-transfer-method (TTM) options for velvet:
TTM for velvet -
https://humble.fish/ttm-for-velvet/ - honestly, I did not know this was a thing. It looks like it would require at least 3 containers to give everything time to dry, perhaps 4 to make cleaning easier. And quite a bit of saltwater, particularly if you're treating larger fish. I think a 20-long is the sweet spot for a TTM tank for large fish, I don't see the benefit of a 29 over a 20-long since they have the same footprint. Regardless, the space you need is only enough for two tanks, so approximately 24"x30" or 12"x60" since only two will be running at any given time. If you were someone that was bringing in many fish fairly often, building a stand for two of these tanks seems like it would be ideal. 7 total tanks full of water, so you're looking at 140g of salt water at 1.021...like 3 bags of plain jane IO, or in other words approximately $20-30 for the peace of mind that you didn't sit your fish in toxic chemicals for 2-4 weeks, regardless of their prevalence of use at LFS (who commonly run less-than-therapeutic levels, like .35). Worth it for some, not for others. But this answers the OPs question.
Experimental TTM with peroxide -
https://www.reef2reef.com/threads/hybrid-ttm-to-treat-all-parasites.640247/ - if this works, it would be quite a bit easier than the "TTM for velvet" option since transfers would be less frequent and uses about half the water.
Chemical treatments
Velvet -
https://www.reef2reef.com/threads/velvet-amyloodinium-ocellatum.217570/ - notice CP is preferred over copper
Ich -
https://www.reef2reef.com/threads/ich-cryptocaryon-irritans.191226/ - treatments include TTM, CP, copper, or hyposalinity, in order of my preference...personally I'd rather use CP or copper over hypo since there are some reports of ich surviving hypo. If you're treating velvet with CP or copper it'll snag ich as well.
CP discussions, including where to buy:
So, I called a bunch of local vets here in the Sacramento area where I live and I can't seem to get any vet to prescribe me Chloroquine phosphate after I explained that they are for marine fish, etc etc.. they need me to "bring in the patient/animal" in order to prescribe it.. Where the heck can...
www.reef2reef.com
Chloroquine Phosphate *** The information contained here is subject to changes as I experiment and learn more about Chloroquine Phosphate *** What It Treats – Marine Ich (Cryptocaryon irritans), Marine Velvet Disease (Amyloodinium ocellatum), Brooklynella hostilis and Uronema marinum. How To...
www.reef2reef.com