Wow, the tank looks awesome! It looks like the land snail shells have some air in them. If you just hold them completely upside down (the opposite of the way they are now), the air should bubble out. It's possible you might need to twist them a little, like you're screwing them down, but usually the air just bubbles out as soon as they're turned upside down.
Is that a water sprite? I love that plants like that because they just go crazy and grow grow grow. I even like the way the little plantlets can float on top of the water with their roots hanging down.
I'm so excited now. I hope you're able to get your multis soon!