Hi guys, sorry that I have not replied faster. A big thank you to all the great comments and especially to Lupin. Do not worry, I know that it is often a misunderstanding about the benefits of green water. I know of some goldfish owners who would put buckets of water in their balcony to sun it so that it becomes green. I think the cichild breed can be trained too, but personally (I guess because I am a girl) so I like to raise the ranchu because it is just more cute and chubby. It is not overfed, I train it slowly over about 1 month period. Hope this clarifies some concerns.
EDIT: The method of training is found in the sidebar of the video.