Fishspawn

I got home last night and found my longfin danio had suddenly died. He was housed with 2 other danios (both fine) and a few guppies. Nitrates, nitrites, ammonia all good, ph a little high but nothing bad. I've noticed there's a lot of fine slime around the sides of the tank lately, annoyingly making it hard for the filter to stay attached. The danio was a tad bloated, floting and i noticed his gills were a tad red as were the points of his body where the fins are. Totally miffed! Any ideas?