My opinion is that clay pots tend to make an aquarium look like a trailer park. No offense to anyone living in one, but I think you know what I mean.
As for your petco cave, not to be negative, I'd recommend trashing it in favor of natural decor. Real rocks, driftwood, etc. If you're broke, I understand. Some LFS charge way too much for rocks and wood.
But if it's super nifty and you want to use it, I would scrub it with water, an old toothbrush, and use a little
clear unscented liquid hand soap (like softsoap from walmart). Most will tell you never use soap, but i feel that this type is okay if completely rinsed. This soap is the only soap my wife (a nurse) and my friend ( a tattooist) recommend for cleaning wounds or fresh tatoos. It is mild and contains no contaminants like pigments or fragrances that are left behind after rinsing.
DO NOT use : dish soap, laundry soap, shampoo, etc. These leave hard to remove harmfull residues. DO NOT use: Scrubbing bubbles, ammonia, clorox, borax, or generally anything chemical. If you don't have the exact type of soap I mentioned, then don't use any. Just scrub under running water with brush, taking care to scrub everywhere.
After the scrub down rinse it off
really well and then boil it for 20 minutes or so using COLD water from tap/faucet to fill a pot and bring it to a boil on stove. Don't bother treating the water you're boiling, it's just getting washed down the drain anyhow. THis boiling process is hot enough that it will kill bacteria and etc.
Carefully remove it after 20 minutes and let it air-dry on a paper towel. Then you're good to go.
If you are seriously concerned about mold, fungal, or bacterial growth and feel that the boiling process may not completely kill it, some people recommend a 10% bleach to 90% water mixure. I do not, and never will. Bleach is a dangerous toxic and deadly chemical that is very hard to completely rinse off even when pre-diluted.
WARNING!
If the cave thingy is plastic, or is painted, cheap, etc. the boiling may warp the plastic or damage paint. Excessive heat will also cause the release of chemicals/toxins from some types of plastics. Usually boiling water isn't hot enough to cause this, but just thowing it out there.
Found these on google image search. Look trashy to me, but opinions are individual and I don't know everything. Maybe you'll be the one to change my mind on the subject.