i used to use just river stones of an inch or more in diameter, but found waste and food got trapped in between them too easily, so needed a filler. gravel - even fine gravel - gets eaten and in my opinion has more of a chance of causing a problem than fine sand. it also depends on how you feed them - i always hand feed, so have very little problem with my axies ground feeding and eating substrate. if you drop food in for them to find, i'd avoid both if at all possible.
i have never used a plant growth thingy - haven't found the need. just get some hardy, well-rooted plants like elodea, and you'll be fine.