Fish: [tropical fish club] dead male guppy

Hi Group

After a bad start I have been doing better with fish, since taking your advice. However early this afternoon I found I had lost another guppy.
Only one male guppy is left now. There was no sign of illness yesterday - I checked on all the fish three or four times and he was fine last night. Unfortunately I had a crisis this morning and didnt check until early afternoon when this fish was dead.

Got the magnifying glass on him and could not see anything obvious apart from the fact his mouth was open and 'pouting' would this give any clues? Another thing I had noticed the last week or so, is the two male guppies were bulling each other and other fish as feeding time. Could I be feeding too little - the last two fish I lost were guppies very close together without any obvious illness, but again I felt the the surving males were bullying a little. I am now down to 9 fish (1 guppy, 3 neons, 1 panda catfish, 3 tiny glowlight tetras, 1 black neon tetra) and 2 armano shrimps. 70 litre tank. Plants thriving.

9 September water readings:
ph 8.2
Amonia 0
Nitrite 0
Nitrate 5 (very good as I had 40+ 17th July before I changed filter media)

The above readings are the same today.

If I loose the other one, is there anything I should be particularly looking at? Or is there anything preventative that I can do?

Sue Francis
Surrey England