Why are my guppies dying?

Every time I buy guppies, they die. I don't believe this is because of disease because there are no obvious symptoms. I usually lose them at the rate of 1 every one or two days until there's only a few that survive. My pH was at 7.0 and KH was at 4�. Since finding out guppies prefer harder and more alkaline water I've changed that to 7.5 and 8� respectively. Unfortunately this hasn't stopped the problem. Ammonia and nitrite levels are at zero and stable. I do a 33% water change every fortnight and this is an established aquarium. I maintain the water temperature at 25�C. I am experienced in aquarium keeping and I am at a loss to figure out why this only happens to my guppies. I keep them with platties and the two species are good community fish. What am I doing wrong??