Oh just saw the last couple pictures now and realized the size of the tank. Sorry to say this, but I sincerely recommend returning/rehoming the SAE. They grow to 5 inches in length and really need a 4 ft wide tank (3ft might work, but still a bit cramped) as they like to have the length to swim in. The SAE just wouldn't really be happy in that size of a tank. And as mentioned, they really are a social fish and like to be in groups of their own kind. And adding more to that size of a tank just would make things worse. SAE have been reported to get a bit grumpy when they are older and that behavior may come even sooner when their environment is too small, not enough of their own kind, making them cranky and he may start messing with the other fish.
You might be interested in Otocinclus catfish, which have the same markings, but stay much smaller, they eat brown algae (diatoms) and some soft green algae, but even they are not recommended for newly set up tanks as they often only eat certain algae, and if there isn't enough food, they will starve to death since they don't typically eat fish food. They too would be better off in a bigger tank, but they would work better than SAE in this tank. And both SAE and Otos prefer higher current waters with higher dissolved oxygen levels to be happiest, and with that usually comes cooler water temperatures (if that's a Betta in there, they are quire different in preferences -ie slow-still, warm water).
Not trying to offend you, just being realistic about it. It would be in the best interest for the SAE, other fish as well as yourself (to avoid the headache of the fish growing agitated) to return/rehome the SAE. He might be ok for right now, but SAE do grow fast, especially when young.