Water quality killing my guppies?

Question
I have had a small 10-gallon tank set up for about 3 months.  I gradually added a final total of 3 male guppies, 2 female guppies and an albino catfish, in order to allow the good bacteria to catch up with the additional load.  For some reason, I have lost 2 of the males and both females over the course of the last 2 months.  I found that the RO/DI water I was using (I use it in my reef tank) was bad for freshwater, so I switched to tap water treated with Stress Coat.  My tap water is very hard, so I do a half-and-half mixture of tap and RO/DI water.
My Ammonia and Nitrite are both zero.  My Nitrate is around 5 to 7.  My pH is around 7.6 to 7.8.  The temperature is around 76-degrees Fahrenheit.
I have a severe hair algae bloom, undoubtedly due to the Nitrate.
Any clues as to what is killing my guppies?  I'll appreciate any and all help/suggestions.
Thanks!
Bob

Answer
pH change.

RO/DI water has a pH or about 6.0. Your tap water, like you stated is MUCH higher. This sudden swing in the pH probably stressed the guppies out a ton.

Your nitrate is not high at all, in freshwater tanks anything under 20 is considered clean. Not like in reefs where if it gets over 2ppm you start freaking out. ;)

It is more likely caused by the phosphates. It should go away though with frequent water changes (50% weekly or 25% twice a week).

-Nick