Algae Problems - looking for some help

[IJC]

My tank is growing a dark green algae at an amazing rate in the past week - not just on the glass, it's even covering the macro algae, and even some of my corals.

In addition, my snails have been extremely docile - none of them are up cleaning the glass like usual, and there have been some deaths.

The fish are fine.

Some of the mushroom corals are looking a bit shriveled compared to usual, as is the xenia.

I reduced my photo period about a month or so ago (to about 6.5 hrs), due to heat problems, so this growth is especially strange.

My parameters are as follows:
PH: 8.1
Ammonia: 0.1
Nitrite: ~0.2
Nitrate: >5ppm

My theory is the water I'm using. I've been doing fine with dechlorinized tap water up til now, but maybe there's something in the water this time of year? More silicates?

Anyone have any ideas? or similar problems with tap water in Vancouver right now?

Any help would be awesome.