The difference in your slab and the pond is that the slab can continue to off gas while you are using it. The pond, after you seal it can not. Using it and sealing it are two different things.
As for the do it yourselfer, that is why we post, to keep you from makeing serious mistakes. You can learn two ways, making the mistake, or learning from those who have already learned from others mistakes.
While sometimes I do make a blunder, I would rather learn from those that have made the mistakes. Its a lot cheaper.
Best Regards
d