Even though goldfish eat algae you can't expect them to keep every bit of your tank clean.How do you deal with all the fish waste? Are you using a filter? Using a UV Filter is a great way of controlling algae AND fish waste. The water passes through the filter where an ultraviolet light instantly disinfects the water and kills algae without chemicals.
I can't say that this Barley straw extract would be safe to use in your stock tank, the manufacturer says it's safe for pets, birds, and other wildlife, but not safe for human consumption. Dr.'s Foster & Smith also carries "Microbe-Lift Concentrated Barley Straw Extract" , which IS safe for fish, humans, and pets. It might stain the water a brownish color in the beginning, but it's not toxic.
There are two products called "Stock Tank Secret Barley Straw Treatment for Livestock Water Tanks" and "Pond Keeper Dissolvable Packets w/ Barley Straw" that you could also use. They're both safe for the fish and animals.
There are also a few colorants to add to both pond and livestock water tanks that prevent algae by turning the water a dark blue, and blocking the light the algae needs. Read the labels, but most of these products are non-toxic to animals and people. A few brands to look for are:
Midnite Blue Lake and Pond Colorant
Airmax Ecosystems Stock Tank Defense
Pond Keeper Blue Pond Dye
I hope that helps!