metoo wrote:
If there is a chance you'd find slate rock or river rock anywhere near where you live, it would cost nothing even. But gotta make sure there is no metals and preferably no limestone in them, and to arrange them in a way they would be stable even with big digging fish.
You can use roofing slate, carefully break it in large rectangular pieces and assemble into box shapes and stick together with aquarium sealer. This is cheap if you can get the slate cheap
If you go for PVC pipes go for the stuff that is at least 4inches in diameter as JD get reasonable big
In my tank i have large rocks on the floor with bogwood to provide the top (just make sure its stable)