Help with advice about poorly frogs

[Lousfrogs]

I have wanted a dart frog for some time, but was unsure about how to care for one. A friend wanted to get rid of some unwanted frogspawn (common frog) from her pond that had too much, so I thought it would be good practice to look after them, and release into a pond we planned to put in our garden. The frogspawn turned to tadpoles, then to froglets and with lots of research and hours of care, I had 45 very happy healthy froglets. I bought a massive 4 foot tank for them, with a 3 inch deep pond at one end and dry land at the other. Suddenly, I lost 3 frogs, even though I clean the tank regularly and nothing changed. My research brought about information about chytrid fungus, so I treated them with a dilution of Lamisil as recommended, daily for 10 days (yes, it was great fun catching 42 frogs every day! not) It seemed to work, as they continued to thrive and grow and no more deaths. Then, about 6 weeks ago, when it was really hot, a frog died every few days. Most had no apparent reason for their death. I am now sadly down to 6 frogs, and I fear at least 3 of them will die very soon. There have been cases of ranavirus in our area, so I guess they could have this? I am trying to make their last few days as comfortable as possible, but have noticed one of the frogs now has a massively swollen right leg. I can't find any reason online for this and am really not sure what to do. One of the frogs still seems very active but 2 of them are quite pale, and are almost pink. When I carefully turn them over, there are lots of blood vessels close to the skin. I researched red leg, and they have been in hospital tanks, individually quarantined for the past couple of weeks. They picked up after I gave them a couple of salt baths (I read this helped online) and started eating again. I bought calci worms (had been feeding them crickets since they morphed) which they loved, but they have been throwing them up after a few hours, or pooing them out undigested. I know this forum is predominantly for exotic frogs, but thought someone might give me some advice. I don't have any specialist vets in my area, and the pet stores aren't massively into reptiles/amphibians. One pet store recommended a copper sulphate solution, but I have heard controversial things about this. I fear they are all dying of some horrible disease, so I just want to make them comfortable, but am really worried about the frog with the swollen leg in particular. On a secondary note, during my research, I have read that common frogs are very susceptible to diseases and infections. This has been a very traumatic experience for us, so I am wondering if keeping a pet exotic frog will be the same? Any advice would be great. Additional note - the frogs don't exhibit any external ulcers or bleeding that leaves the body (via mouth/anus) as per the symptoms described of ranavirus on some websites, so maybe they have something completely different. They are off their food, less active, some appear to have plastic-y skin despite being in water, and sloughing their skin very regularly.