ur losing all your CO2 because of the air pump.
Typically u just want CO2 and only CO2.
The way a yeast works, is as it ferments, it produces co2..
This CO2 then get fed though the straw and into your system.
Also you cant use a diffuser on a DIY... the reason is diffusers mostly require a moderate amount of PSI. And that type of PSI can burst your yeast container.
You want a bubble diffuser... the ones which a step like ladder, which slowly diffuse the co2.. or u want a reactor, something which doesnt require a lot of PSI to be fed.
@ grnbrg, that isnt a diffuser, you have a reactor.
Diffuser uses a ceramic disk, or some form of a very tight pours ceramic.... and forces bubbles as a very fine mist.
You have a in-tank reactor setup the way u have it.