Hello...i have a female guppy with a light-brown patch on the end of her tail...her tail seems to be normal and no jagged appearance...is this a false alarm
yeah does sound a little more like a pattern *crosses fingers*. my experience with fin rot is that it usually doesn't color the fin - it usually makes it more transparent. but I haven't kept gups.
has she been under a lot of stress lately? if you got her young, then this spot could be just developing