Note: If the rot is very slight or mild, you could use Maracyn I and Maracyn II (together at half the dose each) by Mardel. These med can help but are not very powerful. They come in hard tablet which is a real pain to administer. You'll have to crush them into a powder before you can administer to a betta in a bowl. A tablet treats 10 gal of water, so you do the math. Do not overmedicate! If you are caught by surprise and do not have a First Aid Betta Kit (naughty naughty), then you can run to a local Petco or something and probably find the Maracyn I & II and at least try that. My advice though? Be ready! Get your Betta First Aid Kit now so you have the real powerful stuff at hand and are ready when the you-know-what hits the fan! (or life will get real messy LOL).