[ghengis]

it's just simple Co2 snot / gunk accumulating on the leaves.

I use a glass diffuser that collects "snot" around the lip. I can see how this "snot" would be sprayed all over the tank, if I was using a powerhead for a diffuser...I think you should put the sponge back on.

I am not using any check valve between the co2 generator and gas separator bottles or between gas separator and diffuser. Would adding one check valve between either of them fix the problem or is sponge in the outlet the only solution ?

Can, if you like, but I don't think that is the problem... And I don't think a checkvalve will stop the gunk from making it to your powerhead. Could you switch to a glass diffuser?? Might make a difference. Just make sure you clean it regularly.