Planted Tank Guru
Join Date: Mar 2009
Location: Madison, Wisconsin
OK here are more details.
The clay pellets keep the finer riparium gravel from spilling through the holes in the bottom of the planter. You could use a coarse pea gravel in place of them for this purpose.
You can fill up a planter with coarse pea gravel, but be aware that this will add a lot of weight and make the suction cups more likely to slip. I would only recommend this with plants that you want to try to grow that are not true semi-aquatic marginal plants, such as many houseplants. A coarse gravel will allow free water diffusion through the planter and more oxygen in root area and such non-aquatic plants might be able to grow alright.
I do not recommend filling the whole planter with the "hydroton" clay pellets. Since the clay pellets are very light in weight plants will be likely to tip out of the planter. The clay pellets will also wash free very easily and float all around the aquarium.
The best plants for riparium planters are true semi-aquatic marginals! These prosper best in riparium conditions and they also make the best representation of the natural streamside/riverbank/swampy environment. Most of these plants are "heavy root feeder" and will grow best if you fill most of the planter with a finer clay gravel which provides more nutrients via cation exchange.