Number of outlets on air pump affecting air output?

I would say no. If only 1 line is being used, then the excess air that is spread across the other 3 lines would end up being forced through the only open line. A caveat to this is if the pump has internal piping to avoid this situation and would just let the excess air out beneath the unit (for example).

I’d just recommend connecting the 3 closed lines to a single tube and then using that, or more ideally, downsizing the unit to just have the number of open ports you would need to use in your setup.

Nah, bro, it ain't the same. If you got 4 outlets on a pump and only 1 open, the air gonna be forced through that one open line, makin' it way stronger. Unless the pump's got some internal pipes to release the excess air. But honestly, it's better to just connect the closed lines together or get a pump with the right number of open ports for your setup. Keep it simple, man.

Alright, here’s the deal: the number of outlets on your air pump does matter, but it ain’t as simple as “more holes, more air.” If you’ve got a pump with 4 outlets and you only use 1, all the pumped air will just get shoved down that single open line—unless the other 3 are totally blocked off and the pump doesn’t have some weird internal vent or release. Most hobby-grade pumps just split the air equally, and if some outlets are open to nowhere, you’re just dumping air you paid for into the void. Waste of power, if you ask me.

Now, if you use a 1-outlet pump and a gang valve? Technically, you’re splitting the same amount of air, but you get more control over where it goes. However, every extra fitting and valve you add is another spot for leaks, backpressure, or just general inefficiency. Pumps with multiple built-in outlets are usually designed for that setup, so you’re not fighting the hardware.

Bottom line: unused outlets should be blocked. Otherwise, it’s like running your AC with the windows down—pointless. Want max air in one line? Block (or tie together) the others, or just buy the right pump for your setup. No need to overcomplicate things.