swordtail fungus?

Hey,
Just a quick question for some experienced types:
I noticed a few days ago that one of my female swordtails had a bit of white on the very end of the bottom of her tail. Yesterday I noticed that the male has a bit of the same on the end of his 'sword', only accompanied by a bit of a red spot. The red matches his colouring, so that may be the difference (I hope).

Any ideas on what this may be? A fungus perhaps?

There are 3 swordtails in the 10g (2F, 1M) accompanied by 2 albino corydoras.

The tank is almost a year old. Recent changes are increased dosing of KCl for the plants, some of the step plants were showing symptoms indicating K shortage. I also regularly dose traces and Magnesium sulfate. Occasionally add a bit of phosphate of KNO3 if NO3/PO4 test low.

I have changed the substrate gradually over the past several weeks to replace that standard 'aquarium gravel' stuff with #2 filter sand (diameter ~3mm).

Last tests:
NH4/NO2=0
NO3=~5ppm
PO4=~0.25-0.5ppm (both these were bumped up after the test)
KH=100ppm
pH=7.0
GH=100ppm

Thanks Leopardess and OrionGirl,
I think that I'll just sit tight for now and keep a close eye on it. I wasn't thinking about fin nipping before but it makes sense. The male seems pretty eager to breed but sometime the females aren't havin' any of that. I was just worried that it may be spreading. Thanks for calming my nerves a bit!

I'll look into the melafix if the spots start getting bigger, but I try to avoid adding anything to the tank if I can at all help it.