In article , NetMax
writes
It would depend on the heater's wattage.

I haven't worked out if this configuration would be more or less
efficient than the normal way. That would influence the wattage of
heater I would need. It is to be a 220 litre tank, so I guess I would go
with the standard rating for that size (yet to be confirmed) and see how
it goes. I guess if it is too much, the heater just won't come on as
often as it might. I doubt that this method would be less efficient at
heating the water than just having the heater sitting around somewhere
in the tank, but if it were, I could always either replace it, or buy a
second small heater to sit somewhere else in the tank.

This configuration is usually with the heater in the input to the
filter. I guess that mixes the heat better, heats according to tank's
temperature and not filter's temperature, and is a bit more fail-safe
if the filter should fail. I think 200W is getting high enough to melt
plastic.

I only suggested the outlet from the filter as that would be easier in
practice, but I see your point. It would be harder to work out how to
fit it in to the inlet, but it could be done.

I think your idea will work. Test it by turning off the filter for a
few hours, while the heater is running. In this mode, it will not
provide any significant heat to the aquarium. Note that your filter's
pump becomes a heater component, as any interruption in flow causes the
heater to be inadequate. Considering the likelyhood, ymmv.

I'm not planning on switching off the filter for any long periods. Eheim
reckon you only need to clean them every 3-6 months !!

Riskier with planted tanks where leaves can clog the filter's intake,
slowly compromising your ability to heat uniformly. If your intake had
a large grid to capture debris before the siphon, then this is less of
a concern.

Malawi tank, so little or no plants (unfortunately)

Thanx for the reply

--
Alan Silver