I am an avid gardener but new to terrariums. I am hoping you experts can help me. I want to make sure my creation is actually viable for a decent amount of time; maybe at least a couple years? I am trying to create terrariums with a base layer of colored sands arranged artistically. To keep the sand from moving or mixing I saturated that layer with water but am afraid of algae or molds ruining it.

Here are some ideas I've considered:

If I leave the sand dry then when I water the plants there will be distortion of the sand art colors. The colors are darker when wet. Dry sand is also more likely to shift and ruin.

A book I have suggest aquarium algecide in the sand layer. Pretty sure that would kill the plants. (?)

Another idea from a book is to pour hot wax on the sand to create a barrier. Haven't tried that yet. The only impact I can think of is whether this will be visually appealing in a glass vessel.

Like I said the other option is soaking the sand layer and have a gravel layer between the sand and the soil. I like the way this looks, it allows more room for error in watering, but I'm just worried about algae and mold.

I am attaching a picture of my sand art terrarium here for you to see what I'm talking about. Opinions and criticism are appreciated. If this looks like it is not sustainable what else would you suggest? Thanks in advance!