In article ,
says...
I tried to use the system 1 diatom filter on a 20 gallon barb only tank - just
to polish the water - I ran it for 20 minutes - it created quite a turbulence -
I later found out that this filter is only really for larger tanks - 1 day
later - the fish started dying - about 1 fish per day. I lost all 9 fish
eventually. All parameters are normal
now - but I wonder if there was a sudden ph change or what happened ?

Did you know your water parameters before, and check them afterword? I
haven't ever used the filter you are talking about, but I doubt it was
the problem. Did you do a large water change recently. Did any of the
powder from the filter get into the tank, I've heard of that causing
problems with the respiratory functions of the fish?

Josh