Hay, sorry to see how sick your boy is. I think if it is what Mr.Vampire says it is, theres only two out comes. One to do the treatment he suggested or two, be ready for your little man to pass on soon. On your half what you can do to help him to fight off the infection is buy a heater for his tank. Look for one that is around 10 watts and of course it will say how many gallons its for. Long as its preset or you can set it to 78*F. Also For the next 10 days do 100% water changes with conditioned tap water and a daily dose of his medication. Be shure you read the labels to the additives you use and never add more then what is needed even with the Tap water Conditioner. I hope he gets better soon and hows your surrority btw?
Oh noo. :'( She musta been sick with something undetectable. Is she one of the ones you just purchased or was she one you had already because most petstores have a 3 day fish death pollacy and you can exchange her for another.
What I do when I change my water is get a large bucket and use it only for your tank so chemicles or cleaners arn't used in it. Or a cooking pot to do small changes same with making shure its just for your tank. Or you can get something called a gravel vacuum witch many you can hook up to your sink or syphon into a bucket like the one I have.