These are baby Pot-Bellied Seahorses that are native to the coastal waters of Australia. This species is famous among seahorse breeders and is one of the most selected species to be brought up as pets. These babies may not look like much, but when they grow up, they would be considered as the largest seahorse species in the world.
Seahorses are very shy creatures. Most of their kind live in dense underwater foliage to hide themselves from danger and predators. An avid fish collector would know that these creatures need a secluded and private place separated from other fishes living in the tank in order for their natural behavior to shine. Removing or reducing their hiding spots will cause in passiveness of the seahorses. In other words, the more you put in plants for them to hide in, the more you will see them moving around the tank.