Hello!
Ok so I took a water sample into my LFS and they said the ammonia was at 0.25 percent, which he said was fine, he also said nitrates and nitrites were fine (forgot to get exact numbers, oops) so I described the fish's symptoms prior to death and he said it sounds like some kind of fungus. so he gave me some all over medi cure I think it was called. Turned the whole tank green which looked cool. Just before that I did a 25% water change. Hopefully all goes well! Thanks everyone!!
One last thing, I did buy a gravel vac, but I dont think I'm using it properly. I pumped it up and down to create a vacuum, and a little bit of water came up, but I didn't see any bits of faeces or anything come up. I think it may be because the water level wasn't much higher than the top part of the gravel vac. Maybe i need to get a smaller one or something.