Roemgie your girls are so cute i hope you will get rid of the velvet completely.
If you using the siphon for your 20 gall tank or any other tank, or anything you are using to do water changes (a net, a cup etc) make sure you rinse it with hot water and let it sit outside to dry out completely. Parasites are die without host, so when you dry everything out they die. And the siphon hand it vertically so all the water drips out.
For your 20 gall how i understood you was treating with Maracyn. I don't think that maracyn will help though. You don't want to medicate fish with wrong medications. I understand you can't do 100% water changes for 20 gall you, but what you can do is raise the temperature and do 50% water changes every other day or even every 2 days .
How are they doing?