red minor tetras in corner

Lost one yesterday. It was in the top corner of the tank for awhile
then belly up this morning. Second of three is in same corner. Any
ideas?
Water quality is ok. They seem big enough to handle the gold ram and
yellow lab (all juvies).

If this one also makes it's exit, would some type of small gouramis be
ok in my tank? The two keyholes are very peaceful so I don't think
they'll be a problem.


TIA


Larry