You don't give antibiotics for worms, you give antibiotics for bacterial infections. Fishflex is what you can get for antibiotic for dogs off label use. It is Cephalexin.

I never personally had to deal with a case of adult HWs but I know you don't want to kill them all at once or they could block up the heart and kill the dog.

Treatment & prevention of heartworms are 2 different things. Like the 1st reply, be careful if there's are existing heartworms.
Prevention's easier. I just buy ivermectin sheep drench for my dog.You can google around for the dosages.It will vary by weight.
According to our vet, it's now recommended to dose every 25 days for prevention.