why do they keep dying?

I have a 50 gallon tank with angels, otto's, apistogramma cacatoides,
a figure eight and bleeding heart tetra's.
since 2-3 months, every two weeks one fish dies. I've lost 1 Boekhlei,
2 angelfish and one apistograma that way. They have no symptoms
untill their last hour. Then they sink to the bottom, start havy
breathing ans then they die.
They eat well. I've noticed light coloured stool with a bit of a slimy
aspect. I thought about hexamita but then again, they don't have hole
in the head symptoms and they did not lost weight and eat well, and
they are brightky coloured.
Can hexamita be the reason, killing them one by one without others
symptoms? Should I give them something like hexamita(R) from 'Esha
Labs'? I don't like to do it because Apistogramma's dont't love
medication... But if that's what I got to do...

nay help is appreciated

greetz