hydrogen peroxide can definitely help. i'd say weekly a couple tsp per gal should be more than enough, don't over do it. since you're growing outside the most likely cause of the algae is from water temperature. once it gets to a certain point it becomes a smelly swamp, especially if the container is not sterile.. make sure you clean it with diluted bleach when you replace the water otherwise any trace of previous bacteria will encourage it to continue re growing back.

another thing, if possible, try to add an air stone or a pump to keep the water moving (even if it's only for a few hours per day).. but i guess this kind of defeats the purpose of kratky DWC.

Thanks
Stevie! I waited for a couple of days
and got no reply so I went ahead and took the chance, flushing the roots with a
hydrogen peroxide solution. The bucket was easy to clean but cleaning the algae
off the roots was tricky. I hope I
didn't do too much and end up killing the plant. It's been only 24 hours, it's not out of the
woods yet and I am keeping my fingers crossed.

Calgary doesn't
get very hot so I think it's the new bucket that allowed too much light through
even with a bucket skirt. I had another
hydro tomato plant sitting right next to this one, it's in a smaller bucket
with a bucket skirt, but it showed no signs of algae at all.

I cleaned
and painted a new bucket, followed by duct tape and covered with a bucket
skirt. If that didn't do that trick I
wouldn't know what else to do.

We have
only 3 weeks left before the first frost, the hydro plants will have to come inside
the house. They will receive an air
stone each, once they move into my bathroom.
Not a long term thing, I just want to buy them time to finish their
season.

Calgary is
not the best place to grow anything.
Last frost around May 23 while first front around Sept 15, and we
had snow a week ago, almost killed my plants, so go figure.

when you have black container it blocks light, but then you also create another problem.. black absorbs almost all the suns heat and heats up the water. it's not so much the air temp that matters.. if i use a dark container outdoors i will typically cover it with a white plastic bag to reflect the light/heat. the water can be like 20 degree cooler by doing this..

also, you can buy white plastic grow bags for cheap. then use this material to cover your container. they look like this: http://htgsupply.com/Category-Grow-Bags

the big bags can be cut and taped to the container, or for buckets just buy the bag depending on bucket size and just slip the bucket right in the bag. they work great to not only reflect the heat because they are white on the outside, but inside they are black to block the light as well. one stone kills two birds ;)

cheers.

Thanks for the tips. The same reason I put a bucket skirt over the bucket, to keep the bucket cool. The Panda film is white on one side and black on the other, just like what you said.