eheim jager

I really need to know if anyone has experience with placing your heaters in a horizontal position in your sump? The instructions say vertical for my EHEIM JÄGER 300W (model 3619)

Hi guys, I just recently purchased a 125W of the whisk Hager heater for my 40 gallon tank running with a 10 gallon sump. I thought a 125W heater should be enought because it is for 40~53gallon. And I set the temperature at 82°F for the first day and let it run for 24 hours, but my thermometer...