I am new to fish keeping, I have a Juwel Rio 125 tank, with 6 CoryDoras, who are very happy, and 8 Cardinal Tetras, who are mostly happy, but have their moments where a couple of them want to chase the others away from their territory in the middle of the tank. I think this is usually after I've been fiddling with the tank, - water change, add something new etc.

I did wonder whether this behaviour was due to low oxygen, and have since read that live plants that dont have much sunshine can actually reduce the amount of oxygen in the tank, - and that the plants need light to produce oxygen?

My thinking is to put the aquarium light on for a couple of hours after the room is pretty light anyway, to reduce the sudden contrast.

Also - my bunched weed (?), and Echinodorus are planted in my sand, is this okay? My Anubias's roots are wrapped around a spider wood root. Do I need any additional fertilizer, - shop said no.

Any thoughts and advice would be much appreciated, thanks.

As long as the surface of the water is rippling there will be adequate gas exchange so not a problem. If you find a couple of the cardinals being nippy with the rest, consider increasing their shoal size as that can help.

Regarding fertilizer, the Amazon sword plant needs feeding at the root so if you haven't got a planting substrate (IE you have plain sand or gravel) tuck a root tab in there under its roots. The others shouldn't need anything.