Does an external filter have to be underneath the tank or is it able to function just the same next to the tank?
That’s good to no, the filter ie Eheim 2213Makes little to no difference for most setups, having it lower than tank will make it easier/quicker to prime as a siphon works better with a bigger head of water, once the filter is primed the shorter to pipework/hose the less the resistance to flow.
So minor pros and cons in most cases
Exactly like @ian_m says. I have my filters <"sitting next to the tanks">. Eheim Classics don't have a <"large head height requirement">, but you do need to keep the hoses clean etc. or you will have problems with noise and filter damage.Most external filters will have requirements like this.
Most external filters will have requirements like this.
View attachment 153862