No, you've learned a valuable lesson. What is good for one thing is bad for another. Time to get back on the horse & ride.

Most plants do poorly with salt. It is usually better to get rid of the livestock, that try to deal with disease. I've never seen a case where treatment did any good. Fish get sick when their defences are down, just like us. Until the cycle issue is taken care of, nothing will help.

What I would suggest is that you get your temperatures up a bit and re-start your system with something really tough, like gambusia or black mollies. Black Mollies are especially tough and are commonly used to get a cycle going. Gambusia are also known as mosquito fish and are a big guppy. Gambusia also have a good market for pond folk as they do a great job in controlling mosquitoes. They can over-winter as well.

Mollies and gambusia need plant cover, like najas to have babies survive. Parents eat their young.