Phosphate can come from
food, additives to the tank (some carbon) and waste from creatures in the tank.
Your on the right track to elevate the calcium level, this will lower
phosphate. Also, you don't say how old the tank is? but if its fairly
new (less than one year) the algae blooms are just something that has to
be balanced out.
1 PPM is not that bad, and I'm sure if you follow the suggestions above,
you can get it down