It'd be a really bad idea to just load your dogs up on meds. The risk of these parasites varies greatly by geographical region. For example, in the pacific northwest, there's no need to protect against heartworms.
Is the med X legal in your country?, dunno, but you can order here your dog drugs. Can I mix X and Y?, dunno, order both and read the contraindications later. Are this meds dangerous for my other pets like my "always-chewing-dog-ears" cat, or my ferret? this web don't give you any clue. I know cases of kitten killed by the wrong treatment; Can you calculate the right doses for your big but starved and weak new rescued pet?... etc, etc...
Of course the owner can and should take some time to educate itself about recognising diseases (and much specially poisonings) of his pet, but good and comprehensive manuals are available in your book store, and of course to identify what kind of roundworm could or not have your dog is not trivial at all for a normal owner.
The guy that that puts all in his tank to cure "don't know what's this thing growing in my fishes" is sadly very common in aquariums. He/she kills currently much more aquarium fishes than the real diseases do.