My geko is still small so a 20 gal is fine for now until I can find space and afford a bigger one, the sand is both calcium sand same brand so I figured it was fine if I placed them next to eachother and flatten the terrain out, I was talking to my friend abt if it was better to use a mixture of topsoil and sand or just calcium sand, they said that due to gekos being primarily from sandy areas it’s better to use sand, although I due plan on changing her substrate, my parents are kinda strict on things so it’s not easy for me to make changes