dead Guppy :( help needed please

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?

 

HI catbeenie (love your name!) Welcome to the forum

First of all, what size is your tank? Did the tank come with any fish or were they all new? Do you have any plants or decor?

And did you mean to say you'd never had a reading for nitrites rather than nitrates?

 

Thank you! Ha, yes, I always get the nitrates and nitrates the wrong way 'round but I know the parameters are good! It's a 20 gallon tank. We don't have any real plants, all plastic, some that were inherited, some new that I washed in old fish tank water before adding. We also have a few stone and a bridge where they can hide.

 

And the tank came with the pleco she was 10 inches long but I got her quickly to a 55 gallon.

 

And of course now I see at the very top where you mentioned the tank size in the title

Well, hopefully someone with more experience will be along with better ideas but if none of your other fish are having problems I'm going to guess it was a problem just with that fish. Do you have a quarantine tank for new acquisitions?

 

No we only have the one tank but have acclimatised the fish in the bag before adding them and have never put fish shop water in the tank. Thanks for your response xx

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That's good! I'm just thinking for future fish you might want to have a QT on hand so you can observe the new ones for a while before adding them to your main tank. Can help to reduce the chances of them passing disease to your existing fish