Help - Half My Goldfish Suddenly Died

[fishnovice]

Got home Sunday night to find 4 dead goldfish floating on the surface -shubunkin, 2 moors and a common. They were in a large outdoor tank, well cycled with very good filtration and had been living there happily for a few months. On saturday night I noticed goldfish were all gathered at one end of the tank furthest from the sun and several were lying on the bottom instead of constantly looking for food as usual. I thought they were asleep as it was late but I guess they were ill. Moved surviving goldfish indoors to a spare tank with 100% new clean water with a filter borrowed from my turtle tank as i was afraid there may be something toxic in the water which could have contaminated their filter and dosed them with ESHA 2000.Really worried the rest may not make it.
What could have caused it?
-Hot weather, temp in goldfish tank was 29degrees
-Black hairy algae which has suddenly appeared and covered all rocks and artificial plants. What is it and is it toxic to fish?
I really need to move the goldfish back into their former tank as the temporary indoor tank is far too small. If the black algae caused problem how do I get rid of it, remove all toxins and make the tank safe again?
UPDATE
Surviving goldfish have recovered and are swimming actively and eating normally.Tested water in outside tank -zero ammonia and nitrite.
What is that black algae stuff?
Could it have killed the fish?
How do I get rid of it?

 

[Assaye]

How large was this tank? Even if it was technically big enough for the fish, in the hot weather and with the sun beating down the fish could easily have overheated. Unless you can keep the tank at a safe temperature, they are best in a huge body of water (such as a pond) that will maintain cooler temperatures and won't be "cooked" by the glass, or kept indoors in the shade.

Do a Google search or check in the planted section for information about the specifc kind of algae. Sounds like black brush algae though and that isn't toxic as far as I know.