Sparkling gourami suddenly struggling
Came back from work today and noticed that my sparkling gourami seems to have become very weak. I'm not quite sure what exactly the problem is, but I think I know why. Last weekend I got some new fish and stupidly decided to dump them all into my nano tank. Those fish pretty much died overnight. They probably had something pretty nasty in them that spread to my gourami. I'm guessing it's a parasite of some sort.
I am not sure how to treat them as I've never really had to deal with sick fish before but am thinking that I'll probably go ahead and get some API General Cure tomorrow and treat the entire tank with that. The only thing I wanted to be sure about is whether General Cure is safe for plants and inverts and whether it will kill my filter. I think the main ingredients in General Cure are metronidazole and praziquantel.
Appreciate your replies and definitely will not be adding any new fish without quarantining them in a separate tank for a couple of weeks from now on.