Mass death in freshwater tank after adding fish
A few months ago I got two sunrise platies that seemed healthy, but within 2 weeks the first one randomly died. About a week later the second one died. After it died I noticed it had reddish gills and a red mark on its stomach. Then, I noticed three of my other fish had reddish gills and one was pretty bloated (a sword tail, molly, and a platy). I figured it could be dropsy, and did a full Melafix treatment over a week's time. Their condition wasn't improving, so about a week later I did an API general cure treatment, moved them to nurseries, fed them peas, etc.
I kept up with regular water changes and tested the water parameters regularly (using a test kit, not strips) and they were always fine. Eventually, those three died within a few days of each other. This was about two weeks ago. At this point I had three big fish left who did not exhibit any physical or behavioral symptoms (in addition to 5 babies that had recently been born). Last night, one more molly died that didn't appear sick at all. However, this was after a water change that I'm thinking may have been a bit too much.
All I have left are two mollies and my 5 babies. I'm not sure what to do because I want to add more fish but I'm afraid that whatever the sunrise platies introduced into the tank isn't gone. I was hopeful it had been taken care of and that those that died afterwards were already too far gone to recover, but now that one died randomly last night I'm worried. What else can I do to ensure my tank is healthy before adding new fish without potentially harming the remaining fish and fry? Or is it inevitable that the remaining fish are going to die too? Thanks so much for any help and input.