I have a 20 tall set up with 4 columbian tetras, 4 gold columbian tetras, six red fin tetras and 2 siamese algae eaters. all of the columbians are 1-1.5" and the red fins about .5''. My question is why is one of the gold columbians seem to be sinking like a broken ship and swimming at an angle up all the time. This has just started happening about a week ago. It's back end is falling down it seems and it cannot just suspend, whenever it stops swimming up it just sinks strait to the bottom. Any ideas? Ever heard of something like this?