In Fishdom, you use your mouse to swap two items so as to make a row of at least three. When you create this, those three or more disappear. The object to to match items where at least one is on a gold tile. When all the gold tiles are cleared from the puzzle, the level is complete. You will then win money based on how many gold tiles there were and any time you had remaining. Then, you can use this money to buy various fish, decorative items like plants and shells or items for your fish's comfort like heaters, lights and filters so as to create your dream aquarium.
If you match five of the same item, you will create a depth charge. Double-clicking or swapping this will make it explode. It will then clear the area up to two tiles away. If you match six of the same item, you will create a firecracker. Double-clicking or swapping this will make it explode. It will then clear the area up to one tile away. If you have gold tiles with a gray plate, you must match them twice to clear them. If you have any tiles that are locked, with a chain on it, you must match it to unlock it but you cannot move the item within until it is unlocked. If a tile is double locked, with chains crossed, you must match it twice to unlock it.