As you say, dropsy itself isn't a disease, and isn't contagious - it's the visible symptoms that the fish's organs are failing, sadly. But if several fish are dying from organ failure, then there's obviously something like a viral or bacterial infection that's spreading and causing the organ failure and losses.
The best thing to do is what you're already doing. Remove any fish showing symptoms of illness. Euthanising is usually the most humane thing to do when it's dropsy and you know there is something spreading and want to save the remaining fish. A viral, parasitic or bacterial infection can be spread or made worse if the remaining fish eat the dead body of the dying fish, so that's one reason for isolating and removing any fish that are showing symptoms, and euthanising if it's progressed to dropsy. You're already doing that.
You aren't doomed to lose them all, but when a disease is spreading and we can't discover whether it's viral or bacterial, or which bacteria it might be, whether gram positive or gram negative etc, and when antibiotics are (rightfully) difficult to get hold of in Canada anyway... throwing in meds without even knowing what you're trying to treat is often a common last ditch attempt to try to save the remaining fish, but often make things worse rather than better.
Trying salt treatment was fine and a natural move, but what was the other product? Link please? The product name is too generic for me to find the one you're using; I only get results for products for humans. Remember that clean, fresh water is the best medicine in almost any situation. It reduces stress, boosts the fish's immune system so they have a better chance of fighting off whatever is ailing them, and when you wipe down the glass, clean the substrate and do water changes, you're also removing a lot of the harmful disease organisms and parasites, so the fish have less bacteria/parasites in the tank to fight off. So, resist the urge to buy snake oil products as a last-ditch effort, since all medicines cause stress on the fish and the body, and can make things worse rather than better, and there are a lot of useless and potentially harmful snake oil products out there.
Anytime you're feeling helpless and hopeless and want to
do something to try to save the remaining fish, do a large water change, make sure to only rinse filter media in tank water and not under the tap (I'm concerned that there are zero nitrAtes, is this a new set up? Have you been cleaning filter media under the tap?) and take time to sit and observe the fish. Make notes of any unusual behaviour, markings, white stringy poop from any of them, fin-clamping, symptoms of illness etc. Observing these things may help to discover what is causing the losses, and catch which fish are becoming ill and isolate them before it gets worse, etc. The water changes/tank cleaning and observations are more useful than any snake oil product you can get.
If you can get some clear photos of the tank, the sickly fish, and the remaining fish, that can be helpful! I'm sorry you're going through this, it's rough.
So that would be my advice, I'm sorry it isn't more precise! But without lab testing, and none of us are fish vets, we just have to learn what we can, and do the best we can to try to help the remaining fish.
My questions would be: How long has the tank been set up? When were the fish added, and have you added any new ones recently that might have bought in a disease of some kind?
How do you usually clean filter media? Do you replace cartridges or rinse media in tap water? The zero nitrAtes have me wondering about the cycle. A cycling problem/ammonia or nitrite doesn't usually cause dropsy, so not suspecting that really, but if the tank isn't properly cycled yet or is having mini cycles, that would increase stress and leave the fish more vulnerable to other infections.
A request for photos, if possible, please.
