This just started last night. I have had my shark for 1 week today. I looked over & noticed that he was in a semi-headstand & got worried. I checked my pH and ammonia. ammonia at 0. good. pH was super low. around 5. I quickly did a partial water change & added some pH riser to achieve a stable level. None of my other fish seemed affected. Now it's normal, and he's still acting strange. Still in a headstand (not complete vertical) and possibly a little bit to his side, unless someone bothers him & he swims away. Advice?