The 120x120 is the tent size with the most choice, and that is the actual task here. Three diameters all fit it, so the question is not whether something works but which route you take. They differ mostly in what you end up buying afterwards.
Ø 125 mm is the cheapest route
Ø 125 mm has the widest fan choice in the whole range and, contrary to what this page said for a long time, a wide choice of carbon filters too. So you need no reducer to a larger filter; fan and filter come in the same size. For a 120x120 that is not packed and does not sit in a warm room, this is the cheapest complete build.
Ø 150 mm gives the most headroom
Ø 150 mm is the largest diameter group here and the safest choice once both a filter and a silencer have to go on. The capacities spread widely, from the pre-damped Can-Fan Iso-Max 150 to the heavier Max Fan Pro 150 and the RK 150L, so you can size up and run it turned down. It is also the size where AC Infinity supplies a complete system, from fan to filter kit.
Ø 160 mm if you are already on it
Ø 160 mm fits this tent too and is better provisioned than it is usually given credit for: filters and ducting are plentiful, the fan choice narrower but covering every type. Building new, 125 or 150 give you more to choose from. If you already own equipment at 160, stay on it rather than reducing everywhere.
Noise is a real consideration at this size
A 120x120 often stands in a house, and at this airflow you hear a fan. All three diameters have silencers, and acoustic ducting is an alternative that often does more over a long run. Count the silencer into your calculation rather than hanging it on afterwards, because it adds resistance too.
Frequently asked questions
Should I put a 150 filter on a 125 fan?
No. There are plenty of filters in Ø125, so keep the whole chain on one size. Every reducer costs pressure.
Which of the three diameters should I take?
Ø125 if the tent is not packed and you are watching price, Ø150 once a filter and a silencer go on or the room runs warm, Ø160 if your equipment is already there.
Is one circulation fan enough?
Over this floor area usually not. Two small ones, above and below the canopy, move more air than one large one blowing at a wall.
