Aquarium decor advice

[Dranix Paremoon]

My friend just got herself a Betta with a little betta home set. She named it Magneto and I joked about him having a magneto helmet to swim around in. Then I realized I could do that and made one that night out of grey sculpy clay. I painted it with acrylic paint(it was the kind I had on hand. I don't paint with any other kind usually. I'm excited to give it to her, BUT I'm sure I need some kind of sealant on it so the water chemistry doesn't go weird from the paint of clay.

Point of all this:
Is there a sealant or finish or something that is safe to use one a homemade decoration going into a fish tank? Tried looking it up, but it's proving hard.