I had 6 male guppies all from the same store. I have had my setup for 4 months (tank was cycled for 1 week using beginner bacteria) 1 died (which was expected). All fish were doing fine until 3 weeks ago, now each week another is dying all with different symptoms if any at all and there doesn't appear to be signs of infection.
My tank is 57 litres, it is the fluvel flex (15 US gallons) Water conditions are Ph- 7.8 Ammonia- 0.25ppm Nitrites- 0ppm Nitrates- 0ppm I am using API master test kit
Note I also have live plants that seem to be wilting, I do 10% water change each week, I have also tried salts and syphoning gravel.
The presence of 0.25 ammonia and 0 nitrates suggests the tank wasn't properly cycled - efficacy of beneficial bacteria in a bottle is very hit-and-miss (in contrast to fishless cycle using ammonia). [Ensure you shake nitrate bottle very hard before using as sometimes inaccurate.] https://www.fishkeeping.co.uk/articles ... ammonia-nitrite-spike.htm might be helpful.
Treatment-wise, some fish tolerate salt better than others. Your best option is to keep ammonia/nitrite/nitrates at 0/0/20, and increase water changes to 25% to ensure getting rid of things water quality tests can't measure (eg stress hormones).
PH isn't always indicative of water hardness levels - guppies don't do well in soft water which may be another possibility accounting for the problem. Check your water supplier website for your postcode to find out what it is. If you've limescale in your kettle, though, that indicates hard water which guppies need.