Found my guppy nose up in plant and a funny colour help!

[Cocoa19]

Hey! So I got this guppy for my brother about 3 months ago now and I just found him nose up in a plant dead very sad brother, I got him with 1 other guppy who is fine but when I got him he was acting strange straight away, when I got him he was black then he turned white and this carried on I asked the guy I got him from and he said it could be females because they change colour... It's a male, could be water.. everything was fine but I did notice it was when I would feed him he would change colour. He was fine otherwise and then about 4 weeks ago he started going for 1 of my Black Neon Tetras anal fin not nipping but annoying him, only the bigger one and i am lost to why and hes been doing it ever since and then I find him dead so assumed maybe the Tetra had enough, but no wounds just black with 2 coloured patches on him that were whitey blue and eyes solid black, he also acting funny with water changes everything would be fine in the water but he would try and jump out so I started doing less changes from 1 a week to 1 every 2 weeks and he was better until I did a water change again, the way he swum and everything was fine though so I'm at a loss! Was eating fine too, I have 1 other Male Guppy, 1 female Platy, 3 Neon Tetra and 3 Black Neon Tetras in the tank. I just want to know am I going to lose anymore fish or not and give my brother an answer on what happened to his fishy

 

[snork6]

The fish looks normal from the pictures. No obvious signs of any of the disease I know of. What is the parameters of your water? How many gallons/liters is the tank?

 

[Cocoa19]

I just tested my PH which is 7.5, when I checked everything else 2 weeks ago all was normal but getting checked again later today, all my other fish were always unfazed though he was the only one strange and always changed colour and I found it weird how he was jet black with 2 distinctive spots of colour as he was never like that always a mixture of colours such as in the photos, my tank is 26 Liters. I also have a snail in the tank