The three spaces to measure
Three measurements decide the fit: the door opening the filter must pass through, the space inside where it sits, and either the space above it to open the lid and lift the media baskets out, or room to lift the closed canister back out through the door. Fluval’s 07 manual takes the second route: close the AquaStop valve, unplug, release the valve and carry the canister, upright and by its body, to a flat surface near a sink. Hinges, a center brace and shelves often make the opening smaller than the cabinet looks.
Compare those numbers with the exact model. The Fluval 107 and 307 share a design but differ in height and footprint, so one can fit where the other cannot.
Room for hoses and valves
The canister’s height is not all the room it needs. The hoses leave from the top, so leave space above the head for them to curve without kinking and for your hand to reach the valves. The hoses then run up the back to the tank rim: check there is a gap or a hole for them, and keep to any limit the manual gives for how far below the tank the canister can sit. A door that only closes by pinching a hose is not a fit.
Room to clean it
You will be cleaning a full, wet filter, not an empty box. Plan where it goes: a towel on the cabinet floor, room to close the valves and lift the canister out upright, and a flat place near a sink to open it. If you cannot get it out without moving the tank or bending the hoses hard, put it somewhere else, or choose a hang-on-back. Cleaning a canister
The stand is a separate check
This guide measures the space for the filter, not whether the stand carries the tank. Use a stand built for your tank and its filled weight, and do not cut out panels or braces to make room for a filter. For a large tank on an upper floor, ask a structural engineer. Choosing a stand
Compare filter types if a canister does not fit the space you have.