How about some java fern & anubias for plants? They are not planted in substrate but tied with thread or gel super glued to wood (real or fake) or rocks. They both come in different leaf shapes & don't need fertilizer.
As for your big substrate you might try a fine black gravel at 1 end & the coarse at the other. That way you could have some cute bottom fish if you feed at the finer end. Just an idea, there would be some mixing but it's all black & cheaper than replacing all.. Most corydoras catfish or (1 of my fav) botia striata (zebra loach), in a group of at least 6 or 8.