The meds will probably kill ur shimps... A water change might just save them. if your sheimps are still alive, take the betta out and treat him separately in a QT tank. since velvet is highly contagious anyway, you might as well starve the parasite out in your tank whole curing the betta in qt. just a suggestion. some also wouldnt recommend salt with medication, the salt you are looking for is aquarium salt, usually by API, they are very cheap. the dosage is 1tsp/gal to up to 3 tsp/gal ( increased slowly from 1 tsp/gal), used no more than 2 weeks.