HI everyone. 7 weeks ago bought our tank second hand, it had been emptied that day and was set up with wet, used filter, gravel etc and a common pleco that we adopted. We had the pleco for a few weeks but recently rehomed her as our tank was too small. I've been using the API master kit to regularly test the water and even though our tap water is 7ph, we must have some stones that make it alkaline as it stays at around 7.6-7.8ph. The water temp is 25 degrees c and we have never had any nitrates, nitrates sit at 5ppm, ammonia 0. We had 7 guppies for 6 weeks and 2 weeks ago got a baby bristlenose pleco. All fish were happy. 3 days ago, one guppy seemed to be hiding at the bottom of the tank, still doing it the next day.and I notice he had red spots on the tip of his tail but he wasn't bullied and his tail wasn't frayed. I decided to add aquarium salt. I'd only added 2 tablespoons of the solution and unfortunately he died yesterday. When we first got the guppies he was being bullied but this only happened for 2 days and we took the bully back. He was hiding a couple of weeks ago but recovered from this, I kinda thought that the water changes were stressing him out. All other fish seem fine. What was wrong with him and why did he die? I do regular water changes, dechlorinate the water, clean the ornaments in fish water. We have an air stone. They're fed on flakes every day and I put at couple of skinned peas in a couple of times a week. I'm trying so hard to keep them alive and happy, where am I going wrong?