having the phosphate leach from the rock would tank a very very very long time. in my ecology classes it is very well known that the phosphate cycle naturally is the slowest. and that algae blooms are caused by human interaction within the phosphate cycle and not the leaching of phosphate from phosphate rocks.

my thought on the algae blooms caused by adding rock to the tank would be that its actually dead matter on the rock, rather that phosphate leaching from the rock. frequently i have purchased dead rock that has had lichens and mosses on them. also if the rock was once alive but is dead when added to the tank there would be a LOT of dead matter that will be broken down and lead to eutrophication in a sense. also i suspects that even when adding live rock from a store that there would be a lot of die off due to shipping and constantly being handled, things like worms and pods and what not, that when added to the tank follows the same trend mentioned above. and as we know it only takes a little phosphate to make a lot of algae.