If you have an external canister filter, then the output should be at least 1.5 to 2 inches deep into the water. This will submerge it enough to siphon through the water it needs to filter through the filter and back into the tank. For the most part, the size of the filter will let you know. For example, if you have a power filter that clips onto the edge of your tank, then it will sit nicely on the aquarium without you needing to adjust it. On the other hand, some filters are entirely submerged inside the tank’s water. Internal filters such as sponge filters, under gravel filters, and even some canister filters will cling to the side or rest on the bottom of your tank. It will depend on the type of filter you buy and how you want the current for your fish to flow.