Could be a sign of old age, most fancy guppies do not life long, and show signs of "age" after 12-18 months. They live frenetic lives, constantly harassing each other. Don't seem to cause each other damage too much though. Obviously the more they have to swim against the current in the tank the more wear and tear they will get.
1. Lots of water volume- 100 litres per fish 2. Keep temperature around 20-23 deg c 3. Feed high quality high protein foods as well as other green leafy veg etc (plenty been written about this) 4. Well filtered tank with nitrates kept as near to 0 as possible 5. Plants in the tanks for them to feed on.
Where they are raised in the far east, they are kept in large warm shallow ponds where they eat constantly on natural foods and they grow extremely quickly.
I had an eheim 200, but it go noisey after about 2 year which was disappointing, then bought a fluval one... which tbh is total garbage. Given up with air pumps now and have a smaller filter running with a venturi nozzle.