Dropping Like Flies

hi i had 6 gold tuxedo platy fish last october all seemed healthy until last week or at least when the temperature of the UK went crazy. they went off colour slightly and a bit lethargic. Then they have acted a bit drunk and swimming eratically in circles. i do weekly 25% water changes and check the water quality ammonia is really low if at all and nitrite and nitrate is chowing no trace. ive lost 5 now in 3 days and the last one isnt looking so great. The remaining fish neon tetras and moscow blue guppies all are fine. they seem happy and swimming around as normal. Anyone any ideas. The only thing i will add is that the dead fish came from one shop and all the others from another this was all last septmeber/october so ive had them all a while without problems. seems odd its only one type of fish thats going.

The hardness Clark? On the website for my area is 13.63 it says moderately hard. My tank was running at about 30 but has dropped down to 27/28 today.