When using SSR's (solid state relays) an important factor is the holding current.
The holding current is the minimum load current through the triac at which it will conduct with his gate fired up.
The holding current is often specified for 50-60mA (see the datasheet of your SSR), it means that with 110VAC mains voltage you nead a load of at least 5.5 to 6.6W, and with 220VAC mains voltage 11 to 13W. That means that with 220VAC and a Eheim 1046 (5W) or 1048 (10W) for example the SSR wouldn't work...
Also heat isn't really an issue since SSR's are rated for several Amps, and most pumps used in WC only draw a little fraction of that.
CD
Sorry, double post, just wanted to edit it ... :shrug: