I mention platy just because they are a fish that I pretty much ignored for lots of years but then got some and I am really liking them as a "no-stress" item.
Rocks are so often overlooked for such an easy item. Rocks are pretty safe but what may be on the rock can be a problem at times. Things like too much dirt are easy to see and we wash that off but then there may be oil, pesticides or just "something" on the rocks so whether it is wood or rock, I just run them through the same process I would my dishes. I want them clean and sterile, even if I can't see anything on them.
The easy way to do that is a bleach soak as it is simple, easy and safe. Most of the time you just have to ignore all those folks who tell you they have never done it because it isn't safe! I always have to ask how they know it isn't safe if they have not tried it???? Usually they have all kinds of things they have read about on the internet.
But the easy way to make stuff safe to use is the same way tap water is made safe to drink. We add chlorine and let it set for a bit.
Simple? Fill any container with enough to cover the stuff soaking. Add whatever amount of cheap, unscented, uncolored, liquid bleach you can get cheapest. In my case I borrow some from my wife! But it's about $2 a gallon or so and we only need a couple tablespoons or 1/2 cup if it is bigger. We can't say how much is needed because we don't know how much organic stuff we need to react with it. If it's a two foot long log, I use a 1/2 cup but if it is a microscopic bacteria on a rock, tap water may be enough!
Let it set for 6-8 hours or until we get back to the job, rinse it to dilute the bleach down to close to tap water and then let it dry. Let it soak long enough to be sure it penetrates into all the little nooks and crannies on the item. If it's been bobbing around in a ship channel full of oil for long, I want to be sure it gets all of that . When it is done drying, we won't be able to smell chlorine and it is good to use.
The only difference in river rocks that are round and rocks on the land is that some have washed off the hills and been rolling around in the water long enough to wear down to rounded shapes. Same stuff inside.
Neat thing about rocks or wood is that they can let us build shapes and cover them by super gluing on plants like Java fern, etc. to build interesting forms that we can't get with plants alone.