Sick guppy

[Darkesong]

I woke up this morning to one of my guppies in my small (10g) tank floating nose down in the corner. He's (might be a she) an approx 6-8 week old and is in with 2 adult males. He's been listless for several days now and I haven't seen him really eating much. Color is a little off but nothing major, no white spots, gills look fine, the only abnormality I can see is there is a large red spot right in front of the anal fin. I'm fairly new to this so I am assuming its the anus? He is currently lying in whatever manner he seems to land and is frequently upside down. If I swish the net at him he will start swimming but slow and stop within seconds and resuming floating along the bottom. I'm using an aqueon quiet flow 10 filter and when i tested the water today the ammonia was 0 but the nitrites and nitrates were very high. I recently added the 2 grown guppies to the tank so I am assuming the increased bioload caused the spike. I have done a 50% pwc and the 2 adults are swimming around actively. I'm fairly certain lil bit is beyond saving but I would love to narrow down what happened so I can prevent it from now on.

 

[Mouth-Brooder-Fanatic]

i dont think its parameters - from the sounds of it i say internal parasite or infection. time to find out which parasite, and if none match then prob the infection. both can be treated by the heat + salt method, or chemicals from the shop x