A cheap source of caves are those ceramic things you stick in flower pots and fill with water to slowly drain into the soil.
They are already the right shape and size and the ceramic and glazing is tank safe. I just made caves by placing rocks to form natural caves over the river sand substrate I use.
Here is one of mine.
They are already the right shape and size and the ceramic and glazing is tank safe. I just made caves by placing rocks to form natural caves over the river sand substrate I use.
Here is one of mine.
