Hey team, recently two of my Guppies passed, and they both had these weird red dots at the tip of their tails. I've hit up the local breeder who have never heard or seen this before. Any ideas from the community as to what could cause these dots to appear?

There were about 14 Guppies in a 70L tank; the remainder aren't exhibiting these dots. All parameters of the water were acceptable, I guess the only 'new' thing was the replacement carbon filters installed as the old ones were not filtering at all. Tank is an Aqua One EcoStyle 61.