Are you sure it's algae? I have iron bacteria growing in my discharge box from my well and it looks very similar to algae.

Some algaes do like cooler water. In fact, I have algae in the spring in my trout pond but it disapears into the summer and is replaced by chara which is another form of algae that may prefer warmer water.

Could be your well water also provides needed nutrients in various elements that are more conducive to algae's growth.

How big are your brook trout and how large have you grown them? Do you feed them? I have Lake Canadian Lake Nipigon strain brook trout in my pond right now. They are spooky devils and about 14 to 15 inches now. I have grown another strain of brook trout (domestic eastern) to 22 inches and over 5 lbs.

If pigs could fly bacon would be harder to come by and there would be a lot of damaged trees.