Hiya's,
Just read through posts so i thought i'd chime in to maybe save you a few quid (or dollars)
I have had Rena Smart Heaters
They are black plastic/resin looking critters
On the bottom tip is a fish-guard that clips off, once off you have a hole through the length of the heater ..................
Set my moms juwel 240 up with a copy of a tetra external (allpondsolutions) and it's cycled fine (ordered my 1st one yesterday in fact) anyways ..............
I had seen the add-on external fitting for these heaters but thought i'd give a heater a try on my moms external filter
What can i say, took the bottom fish-guard off the rena heater, carefully slipped it over the return feed pipe off the external filter, did a quick bottom clean in her 240, topped tank back up with a 1c lower water temp, switched heater/filter back on etc etc
Bob's ya uncle the rena worked fine, soon started to raise the water temp (left her watching thermometers just in case, she's since called saying all's fine)
I'm sure the pipe diam off the filter is 16mm so after a sliding effort heater was where i wanted it (gotta be 16mm or it wouldn't fit, dohh lol)
So instead of the hydors in my 6x2x2 i'm gonna stick with the tough rena's in 300w flavours and save me a few quid, also if you place them you can still see the thermal warning light if you want to
Might be of help anyways, just saying the rena's work fine externally
Tony