Fish Swimming Against Glass?

[djl08]

Ive read of two different reasons for this,

1) They are trying to produce air bubbles.
2) They are trying to attract my attention to feed them.

Is there any truth in these two possibilities or is it something else. It seems to be mainly my corry's, but my harlequins do it occasionally too.

 

[matchstickman47]

more likely because they expect food, there not begging...
just getting to the front so they get the grub

 

[Geoff1991]

Glass must be strange to them... Theyre probably trying to work out why they cant swim further, around your room and stuff

I dont think its anything to worry about anyway.