Hi I'm very new to keeping fish I originally went to get a gold fish for my kids ended up with a 64L aquarium I started off with 6 harlequin rasbosa and after 1 day they all died so I took a sample of my tank to the shop and they said my water was all perfect including oxygen etc so they give me 5 black phantom tetras as a replacement they all seemed to be doing fine then today after 3 days 1 of thr females has gone and hidden in the log ornament and hasn't come out all day she seems to be sad laid near enough on the bottom no dead she doesn't look ill but then I noticed the other stay in the same place don't seem to wander they swim against the current all day and none of them seem to be eating anything I'm very confused and sad I don't want to loose these fish too as sad as that sounds I really don't know what I'm doing wrong