This is a decorative art technique (faux). I used to do this stuff in rich peoples' homes for a living. Takes a little craftiness, but this technique isn't hard. Prime your tank (or any surface) the way you normally would. Then paint the base color like normal. This is the color that will show through the cracks. Next, cover the base color completely with "sizing." You can buy sizing at craft stores like Michaels, or specialty paint stores. Even Home Depot might have some in their paint section. But here's a little insider tip: Elmer's white glue works too. Sometimes not as well, but in a pinch yes. So you take the sizing (or glue), and using a paintbrush or roller, cover the entire base coated area that you want to crackle. If you just want, say, a stripe down the center of the tank to crackle, just mask off the stripe and roll the glue on there. The sizing will go on white but dry clear. Hint, if you use glue and it's too thick, thin it a little with some water, but don't make it runny. It should have a little viscosity. Once your sizing is dry, paint the top layer of color (the one that will actually crackle) over the sizing. The sizing will prevent the paint from drying as a solid membrane. Once your second layer of paint has dried, use a good clear coat like normal to protect it all. This is one of those techniques that will never be the same twice. You can't really control the crackling, so it will turn out a little different every time. That being said, here's another tip: in general, the thicker the layer of sizing you put on, the bigger and wider the cracks will be. If you put on a very thin layer of sizing you'll get a ton of tiny hair line cracks in the paint. Keep that in mind when you design your paint scheme.
You can find other instructions online or on the side of the bottle of sizing. Just look up "faux crackle technique."
Happy painting.
