It will depend on the amount of water movement arount the heater.
If it is quite still then there is a chance that the heater is warming up only the water immediately surrounding it causing the thermostat to click off. This is more likely to happen if the heater is verticle rather than...
I don't see any good reason why not.
Isn't the 2126 one of the thermo-filters?
Anyhoo...
As long as you can get the media you want for it then it should be fine. It's only a pump and a box when it comes down to it, the same as any filter.
I haven't said anything about what atm should do so how can everything I said be wrong?
If NTD was that contagious then the other fish would already be infected so there would be no point in culling the one fish and if it isn't that contagious there's still no reason.