Anyone can snorkel in deep water, but at a depth of 50 cm it becomes challenging. Of course you won’t drown but you’ll whirl up a lot of sediment and your visibility will be zero. And you’ll run into trouble with your colleagues! Therefore you’ll have to move in a careful crawl - not very photogenic but the best thing for the visibility.
In feed trials it is not important if there are fish present, since their sense of smell is better than ours. The flow conditions are more important. If the current is against us, the escaping food will be driven straight against our diving goggles and then completely out of our field of view. It’s better to stay parallel to the current or, even better, in one of the stagnant zones which are almost always to be found.
As can be seen on the video, the tetras descend on the offered JBL Plankton Pur , which they eat greedily. They do not spit it out once, although they definitely never have eaten this food before. But actually this is not completely true: Most of the tetra species on-site were plankton eaters which feed on the available plankton. A large majority of the natural plankton in the rivers consists of copepods. Even when JBL Plankton Pur contains marine, arctic plankton, the tetras will not taste the difference in such a short time. Even a microscopic differentiation is only possible for experts! Now this explains them gobbling it up, even if the fish are tasting JBL food for the first time. But at least this way they were able to eat their fill for once!