Yes even fish that like soft water need some minerals in the water. I have
found many times when I used to keep SA cichlids that if the water was too
soft they would start to scratch themselves on the rocks until I added some
salt & hard water.

--
Craig Williams
_________________________________

www.Canadiancray.tk
"Mean_Chlorine" wrote in message
om...
I recently did a water change in a tank, where I changed about 1/3 of
the water with laboratory grade RO/DI water.
We're talking water with a conductivity of less than 1 µS/cm, perhaps
1/20th of the conductivity of a hobbyist grade RO/DI unit. Basically
distilled water.

Soon after, I noticed that the sailfin tetras in the aquarium showed
signs of nerve damage - they twitched and convulsed when they tried to
swim. They also stopped eating, and stayed hid for two weeks.

All the while I was going "What the... ?" and trying to figure out
what disease or poison might have struck. Finally, I decided to follow
the old adage "when in trouble or in doubt, do large water changes".

The day after, the sailfins were out moving again, although even
today, two weeks on, they're not fully recovered (perhaps they never
will).

So - what are the symptoms of osmotic stress in fish? Is it possible
that the water simply got too pure, that there was too little
electrolytes in the water for the tetras? Was I supposed to mix some
table salt in the RO/DI water before use?