Fresh water invertebrates query

[Tony Saunder Davies]

I keep rotifers and copepods alive for months in petri dishes.  Fill with 
the algae in which they are collected to a depth of 5 mm or so and expose 
to daylight.  In the case of brackish and marine species (my main 
interest) I have to top up with de-ionised water every few days to 
prevent the salinity rising too far, even with the dishes covered.