I replaced my energy saver sump lights with a 150mh light and the algae is blooming. The tank does not really have algae left in it and the phosphate reading is almost nil. What is causing the strong algae growth then?
Phosphate readings can be very misleading, especially if there is an excess of light in which case the algae will consume it almost immediately. What kind of algae have you got in your sump? It should be a good thing it's growing fast as long as it's not growing in the tank. Just manually remove the algae often and try not to "break" it too much causing it to release the nutrients back into the water column. Eventually assuming there isn't a source of phosphate besides what's currently in the system you should see the growth slow down.
The tank was moved since, but it was green stringy algae. \
The rock was lying outside in the sun for almost three months and the same green algae started soon after the tank was started up. I now wonder if the nutrients does not come from a nenny that died in the tank previously. It died a few weeks before the algae started the first time.