Hi,

I have a 20gal planted tank at work and something strange (to me) happened
this week. Some tank info first;

filter - Fluval 204 (with carbon,biomax, phosgard media) + small rio
recirculation pump
lighting - 2*55W compact fluorescent * 8hrs/day
pH - 6.8
temp - 25C
CO2 system disabled most of the time
plants (probably irrelevant);
Hygrophila difformis
Vallisneria americana
Echinodorus tenellus
Aponogeton ulvaceus
Cryptocoryne albida
Nymphaea lotus
Microsorum pteropus
Riccia fluitans
Fish;
7 harlequin rasbora
3 ottocinclus
20 mostly small guppies

The guppies have been multiplying too rapidly and I've been removing them
(and giving them to my friendly pet store) as often and aggressively as I
can without wrecking the plants.

The 7 Rasbora have been in the tank for over a year (close to 2). Into this
environment I introduced 10 new Rasbora (younger). Over the next 2 days
**all** the 7 older rasbora died. None of the other fish were affected.

Can anyone kindly tell me what happened?

Thanks in advance,

Kevin