The Fluval edge has a HOB filter that sits on top of glass at back of tank under the light unit. Pretty decent little filter as it happens actually.
But wonāt be able to fit a heater in a hob filter at all.
Another potential option is to have an external heater but this calls for a canister or external filter to hook up the heater via the water pipe so that wonāt be a go either for this Fluval edge really.
So the best method in this case is to hide the heater behind some plants and/or decor.
This is what Iāve done on all my tanks, hide the heater behind strategically placed plants