I'm sorry it took me so long to reply. Vet appointment for my furry friends. Anubias is a good low-light plant. Java fern is also good. Hornwort is good if you can find it. Anarcharis sometimes but it's at least worth a try. These will also give your guppies some shelter to hide from each other. Being behind on the water change is probably what caused the rot to happen. I recently had something like that happen to my goldfish tank. 10 days without a water change gave my veiltail some massive rot. If worst comes to worst, you could try to find a large breeder box and give the nipper a time-out. Good luck getting your guppies all healed!
It's definitely not the rasboras. All but one of my guppies are fin-nippers. He had some of his tail missing when I got him (december 2011) and recently it's got worse. I'm behind in my water changes, I didn't do one last week. So, live plants and water changes? Thanks!
I'm not sure about adding salt because I don't know how salt-tolerant rasbora are. Basically treating fin-rot is the same all around. Clean water is going to be your first defense. Try adding an extra water change and some live plants. The other problem I'm seeing is that rasbora can be kinda nippy. If you keep them in groups of 6 or more this will get better. I think if you address the cause of the fin-nipping and keep the water clean you should be good. Hope your little guy gets better!
One of my male guppies has what appears to be a combo of fin-nipping and fin-rot. :P He's in a 10 Gallon with 2 other male guppies, 5 harlequin rasboras and my nerite snail. Is it the same as you'd treat a betta's fin-rot? If so, would the aquarium salt have negative effects on the other fish in the tank? Should I get a hospital tank? I'm going to my LPS soon. I PM'd Sakura but she wasn't on and I have to go soon. :/