How much air......

[Jo and Gra]

Running a 21 gal tank.....

and the th Fluval 2 filter that came with it.

got it running a full tilt......

and got the "air way" open fully at the top to allow loads of air to come in as the water filters......

the other night the filter slowed down, or at least the amount of air did....

the cory started shooting to the top.......and the mollie were at the surface.....

were they low on oxygen already?

how much "air" do they need?

 

[Bloo]

I never use the venturi nozzle as I have a planted tank. You really don't even need that at all.
It more sounds like you have a blockage at the impellor if they output is slowing down. Take the whole filter apart and give the impellor compartment a good rinse under a strong showerhead. (not the filter media of course). These Fluvals do tend to block up a bit so make sure you keep it clean and clear of gunk.

Edit: if they were gasping, do some water checks and see what your ammonia, nitrite & nitrate readings are....