Good evening everyone! I am new to the hobby and just started my very first fish tank this week after wanting to try the hobby for a long time! It is a fully cycled 55 gallon aquarium. Right now there are 15 neon tetras in the tank I purchased 4 days ago. One neon in particular has been isolating itself from the others recently. This particular fish floats in one corner of the tank toward the bottom and will remain there for quite long periods of time. The other 14 neons are quite active schoolers but this fish rarely schools unless it is feeding time or on other rare occasions. There has been some aggression going on in the tank, presumably the tetra establishing their pecking order. I am not sure if this fish was bullied causing social isolation or if I should be worried about disease. As aforementioned, since I am new to the hobby I do not own another tank to create a quarantine tank. I am stuck between taking the fish out and euthanizing it to be safe from neon tetra disease or waiting it out and seeing what happens. Below is a picture. I should also add the ammonia and nitrite is 0 and nitrate is about 20. Thank you in advance.