In article , AQUATIC-STORE.
COM writes
You would need something to circulate water around the tube and create
a flow.

I was thinking of using a tube just a bit wider than the heater. That
way, the water is forced to flow past the heater as it goes down the
tube.

Something like ...

============================
In _________________________
-- |___heater_________________ -- to tank

============================


Where the two == lines are the wide plastic tube and the __ lines are
the heater. The water flows from the filter (just to the left of this
e-mail !!) along the plastic tube, ie around the heater) and out of the
right hand end into the tank.

NetMax suggested having the heater on the water going into the filter
rather than out of it, but the principle is the same.

--
Alan Silver