You can use pottery clay or sculptures clay. Of course there is the problem with needing a kiln to bake the clay. I and probably most of us don't have a kiln or have access to one so instead of using clay that needs to bake i bought bricks of all natural sculpting clay from the dollar store for only 1.25 each! On the package it says the clay dries naturally and turns from grey to white. I will worn you the clay smells like it was just dug up out of the ground.
Anyway in order to sculpt your clay you will need to have a bowl of water beside you the water makes it more pliable and allows you to add peices. I formed the general shape i wanted just by hand and after took a tooth pick and added detailing to make my sunken log look realistic. I let the clay sit over night by that time it was not yet completely dry i could have just let it sit until it dried completely but i was inpatient so i put it in my oven at around 200° keeping an eye on it you don't want to burn it just bake until its dry.
After the peice cooled down i could than paint it. Do not paint the clay while it is hot the paint won't coat it properly. The paint i used was acrylic paint i did a black base coat on it and layered the brown over it so all the notches really stood out than i just added some detail some lighter and darker browns here and there some green moss. The painting is the fun part. Than once your paint has dried completely you can use a clear coat most clear coats will do the same thing but alot of people prefer clear coats for outdoor use you can buy clear coats at any hardware store. I did one coat let it dry completely and than added a second just to be sure i didn't miss anything.
If you can't find sculptures clay or are not sure what to look for you can always use Polymer clay. Polymer clay comes coloured or you can buy just white bricks and paint it. You can bake it in your oven to harden it you don't need to use water while working with it its basically alot like plastercein. Another good think about polymer clay is that if you decide you don't want to paint your peice you don't have to seal it, once you bake it, it basically becomes like pvc. However if you like the shine you get from the clear coat than you can always use it. You can buy polymer clay at art stores or hobby shops.
Hopefully i explained everything well enough if you have any questions please ask. I did my research on these homemade aquarium decorations by searching the web so you can always do the same if you would like to find out more.
I also make my own plastic plants if anyone is interested on knowing how to do that let me know. (saves alot of money!) i don't limit myself to just fish stores for my decorations besides they are so expensive!
any others know any way to save a dollar when it comes to aquarium decor?