you can paint directly on smooth acrylic, just make sure that you paint in really thin layer to avoid paint running. The first couple of layer (3-4) won't cover it all but it would create a good base to others layer to stick on the surface. If you put heavy layer, it won't stick.
Thin layer should only be a little mist of paint really really light. The first and second layer should be practically invisible, it should look like your tank is dirty. You can wait 10 min between each layer if they are thin then after all layer are applied, wait at least 1 day before handling it. Also use water + vinegar to remove any oil that could be on the surface from handling.