Gill Passman wrote:

Nothing added since the Barbs in Jan/Feb....also nothing taken out (apart
from the dead fish). No new decorations or even plants. Very small ammonia
spike (lowest reading) about 9 days ago along with a slightly higher pH
reading (which I'm not convinced I read properly). Rectified all of this and
have not had any suspect readings since a week ago last saturday but lost
another Mollie shortly after. No difference in how I treat the water -
follow the same routine on all of the tanks and there is no problem in any
of the others.

The only thing that I have done differently, a couple of weeks ago, was to
use a magnetic algae scraper. Haven't used it again as the fish kept trying
to get between it and the glass so I decided it was too risky to use it in
the tank.

It it is not disease or agression, I can only think that there is something
toxic in the water that I don't test for. But if that is the case I would
have expected them all to be carried off at once or at least within a few
days but this has not happened.

Here in Canada tanks may get quite warm at this time of year, from the
spring-summer temperature plus the heat of the lights, filter pump etc.
If that's the case you can open the tank lid a bit to let air flow and
cool things down; unplugging the heater can help, too.

When the water quality is suspected as in this case, I think it's ok to
do daily water changes of 30% - 50% for a few days. That will
effectively change most of the water in the tank.

Steve