"Andrew" wrote in message
...
Wuld it be beneficial to my compost heap to add approx 20 to 30
litres of
fish tank water to my heap each week? (its not salt water, just good
old
fish crap and bits of filter contents)

At the moment it goes down the sink, but if its useful I would like
to make
use of it

You are likely to end up with a very soggy heap.

Replacing water in a tropical freashwater tank is an overhyped
activity.
I once kept a 4' by 15" by 15" tank, kitted up with a good filter,
without any water changes for 2 years, just to see what would happen
The fish thrived.

Franz