i would use a wood scraper instead of sand paper when you get to the end or go buy a thin bristle wire brush that can attach to a drill. you know the kind that looks like a wheel with the bristles facing out.
I'd pass at this point, but if you are going to use it, I'd suggest testing it on some fish before putting it in your fav's tank. You could do your best to prep it, then put it in a large tub with water and some inexpensive fish. Add an aerator, but no need to feed the fish. I'd want them to survive at least 4-5 days.
Otherwise there is a non zero risk you'll put poison into your tank.
This is how I feel at this point I just like the look better then a regular clay flower pot I did see some plastic ones and since it's so large id have to cut it to be able to get it in the tank which could be a pain