I have a pair of banggai cardinals that I put in last week. They always pair up at the corner of the tank just staring out. One of it disappear 2 days ago, and this one disappear this afternoon. Are they jumpers? I don't have a lid on the tank. Checked the overflow and its not in there. I do have 1 sally light foot in the tank. I looked into the rock work but it doesn't seem like they are hiding. Where could they have gone?