mass die off

I started with 9 platies; I'm now down to three. I've had at least one platy die each day, a couple days I lost two in just one day. Heated tank (75F), sponge filter, 20 gallons, lightly planted, over 2 years old. Parameters are as just tested today: Alkalinity 40 mg, ph 7.6, nitrate 0, nitrite...

Hi, recently in attemp to cycle a new tank i had mass die off. Initially i had 4 guppies that died within a day and half. From appearance it looked like it could have been the new tank syndrome. Then my lfs recomended putting 3 or 4 glowlites in there which i already have 7 in my other 10 gallon...