You could make things for the aquarium out of clay. For a natural look, use terracotta earthenware. You can also purchase underglazes in various colours to decorate with. All these are available at a potters supplies store. Check with the people in the store that any underglazes you buy do not contain anything toxic, like cobalt. Most of them would be OK.
When making something, ensure that it has a uniform thickness all over, so it doesn't crack in the kiln. Ensure pieces are roughed up and wet before being stuck together. Work your clay well like kneading dough before you do anything with it. Work out any bubbles. Bubbles can cause your piece to explode in the kiln. When your piece is semi dry, smooth out any rough bits, and paint on the underglazes. Even the tiniest bit of clay innocently stuck on your piece can turn lethal when its fired and is hard and sharp. When your piece is bone dry, take it to somewhere that fires pottery and have them fire it for you. You must tell them the exact firing range of the clay you used, which will be written on the bag. You do not need to glaze a decoration for the aqarium, actually it would be better if you don't, so it has a slightly pourous surface for bacteria to colonize.