Which is it ? Many DI units can actually add Phosphates in the IE process. It depends on the active cation resin used. Stick with R/O if possible. The Phosphate can be removed with a Fluidized filter, and Rowaphos, but you run the risk of removing ALL the Phosphates, and that is even worse IMHO since new growth is either absent or corrupted beyond repair.

Phosphates do not directly effect algae blooms. That is a one size fits all marketing myth, that sells alot of product !!! ...Rant Off !

Yes excess phosphate can "contribute" to algae blooms, but only in the absence of good nutrition, and maintenance. Many algaes and bacteria grow just fine in the absence of Phosphates. Plants on the other hand wither, and eventually die. A robust growth rate of plants will always out compete algae in the long run. HTH. Prof M