Yes. It can seriously harm a goldfishes intestines if eaten and can cause fatal diseases to their brain or stomach if its fumes were inhaled. Green Algae is OK for fishes to be around but if excessive algae grows in tank then it could, too, be fatal.
Yes, black algae can be harmful to fish as it can compete with them for oxygen and nutrients, leading to stress and potential health issues.
They are not . I have kept shubunkins and goldfish and they are not harmful to man. They are only harmful to smaller fish such as minnows
plastic plants are not harmful to your goldfish aquarium.
Goldfish are omnivorous and will nibble on a bit of algae as a part of their diet just as people will eat vegetables as a part of their diet.
No, algae are not harmful unless it dies. (Found this answer on Yahoo.com)
snails
No, only black goldfish are black.
Try bottom cleaners, algae eaters, black moors, and guppies for sure.
NO, not usually.
No he's not.
It depends, if your goldfish are smaller than your algae eater, and big enough to fit in it's mouth, then very much so. If the goldfish are small, but too big to fit into your algae eater's mouth, then he might just injure them. But your algae eater shouldn't be so small that your goldfish could eat it. The rule with fish that I always go by is, if it can fit into it's mouth, it will be eaten.
Neither.