If you need a filter that doesn't cost much, just get a mini coke bottle and follow this:
Then the only thing you would need to buy is an airstone/pump. These REALLY work and you can hide them behind decor/plants.
It seriously does well, i've been using them for a while, I only stopped using filters because I was have complications with cycling...
If you don't have a pvc pipe, you can just go without it or find something else for the tunnel that fits and is safe. It would work better with a tunnel.