The strange death of 3 nanacara anomala

We have a well-established (2 years) 325l (~80 us gallon) tank mainly with
South Americans.

The tank is well planted and the plants are growing well.

18 months ago or so we got 4 young Nanacaras (2+2). They all seemd well and
grew, but while we were away on holiday early July one of the females
disappeared.

The last few weeks the alpha-male has lost the appetite and just stayed
behind some bark in the tank, getting worse and worse. Monday i found the
other female dead, and last night the Alpha-male was to bad i put him down.
It was like he was stiff, paralysed, but there was no damage to be seen on
him; he was nice and round with good fins.

The last male seems ok.

Any clues?

The fish were on reduced diet the 2.5 weks we were away.

No other fish seem to have any problems.

Other fish:

10 Cardinals
7 Rummynoses
10 Hatchetfish (sp?)
4 Copper (or is it bronze?) catfish
4 Otocinclus
2 small West African cichlids (2" or so, can't remember sp)
1 Ancistrus
1 Siamese algea eater
20-30 Amano shrimps

Water: Normally 24C/75F, last couple of weeks 27C/81F due to summer temps.
Hardness 18-20dH, pH 7.8 or so.
CO2 from yeast fermenter

Light 11h/d, 4x30W with reflectors.

Food: mostly frozen, bit of flakes and pellets for the bottom. Gave them
live daphnia and migdet larvea back in May, collected from pond which
occasionally dries out (no fish!)

tia

Martin