Red Spot On One Gill, One Fish

[Autonerd]

Just noticed a red spot behind the gill of one of my neons (he happens to be the smallest). The wierd part is it's only the one gill and it's only the one fish -- the other 5 (and the other half of this one) are fine.



I noticed this right after a 5 gal water change, I did not test the water before the change. It's about 1.5 weeks since my last w/c, previous one was 2 weeks before. I have had some problems with plants dying and decaying. You can see some algae on the glass and some debris stirred up from adding water.

Here's the form...

Tank size: 15 gallon
pH: 7.6
ammonia: 0
nitrite: 0
nitrate: 20
kH: Don't know
gH: Don't know
tank temp: 78 degrees
New additions: New (large) plant

Thanks in advance!
Aaron