I measured the pwm signal with a scope, the voltage of the drain with an regular multimeter.
Of course with the 'scope see 0-24V [u]PWM[/u] (or approximately 0V at 0% PWM and 24VDC at 100% PWM). If you're seeing 19VDC the PWM isn't working at all.
Usually the 'scope is referenced to earth ground so you may have to measure the MOSFET's Drain relative to Arduino-ground. In that case it will be "inverted" with 0V when the LEDs on and 24V with the LEDs off.
Im using a led-strip of the following series: 404 Page not found :: Aquatlantis Aquarium - Aquariums, Terrariums & Accessories Manufacturing
There is a special device for dimming this, but it's very limited in its functionalities. So the led-strip must be able to dim, right?
If we only knew how that dimming device works... Does it attach separately to the LED module, or inline with the power supply, or is it a separate power supply?
If it connects separately, and if you have one, maybe you could connect your 'scope and figure-out what it's doing???
There are no detailed specs but since the power supply is a regular (constant voltage) supply, not a special constant-current LED supply, I have to assume there are some active electronics in the enclosure with the LEDs. We don't know how that works, but it would typically be a constant current source and that would "fight" any attempts to dim it. If it were simply LEDs & current limiting resistors it would dim with PWM, but you loose about half of your efficiency.