I just purchased an Ebo-jager 100W heater for my 10g (I know a 50w would be
better for 10g but it's really a backup heater for my primary tank, a 20g)
tank thinking it was a fully submersible heater. when I got it though it
doesn't say anything specifically about being submersible and it has a water
line near the top of the heater. Is it submersible or not?

Also, what is a better way to mount a heater, horizontally or vertically? I
had the impression the horizontal was better but the instructions that came
with my other heater (ViaAqua 100w Titanium tube) say not to put it that
way.