Water quality killing 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
For a 10 gallon tank you should not have more than 1 adult male Guppy. The Guppy that you have remaining has probably killed the other two off. They get very territorial. As for the females, he may have accidently killed them also. The male tends to get very aggressive while trying to breed. The females you have are most likely already pre-hit/pregnant, this is the case for "all" (that I have seen) pet store/shop bought female Guppies. If this being the fact, the females wouldn't accept his invitations to breed so he keeps chasing them, hitting them and stressed them to death.
I keep my male in a separate tank from my two females. Also I separate the fry at one month of age. This way I have virgin females. If you look around, you may find a private breeder that sells virgin females that way you can do selective breeding.
As far as the water, Guppies are very tolerant of different conditions. As long as the water stays under 8.0 PH this should not be a problem. Do frequent water changes, 25 percent every other day for a week, then 25 percent at least once a week to get and keep the Nitrates down. You should think about raising the temp to 78 degrees also. The algae could be from a number of things but you should avoid direct sun light on the tank and adjust the lights on time, no more than 10 hours on especially if they do receive some natural light. (lots of windows = well lit room)
Hope this helps, send a follow up if you have any more questions.