An overflow box allows you to tie a sump filter into an aquarium without having to drill through aquarium glass. If an overflow box fails, though, you can find yourself with a flood, a fire or both. If you use an overflow box, you need to know how to spot trouble.
Blockage can render an overflow box useless. Aquarium conditions naturally encourage growth of algae and fungi in the tubes, which will eventually block them if you don't maintain them. Once a tube is blocked, an aquarium can overflow. Additionally, a plethora of things can block overflow boxes, including wayward substrate, fish and invertebrates. If the overflow box stops, you should check the plumbing and any screens to see if you can find a blockage. You may need to physically pull out the blockage or scrub down the insides with a pipe brush. You can pick up pipe brushes at hardware stores or pet shops.
Overflow boxes often feature siphon tubes to let water flow from the aquarium to the train. The siphon is usually a clear U-tube. However, lots of aquarium equipment produces tiny bubbles called microbubbles that can collect in the U-tube, eventually breaking the siphon and stopping the overflow box from working. You need to regularly check your U-tube for air pockets. This is why most U-tubes are clear.
If your overflow is slow from the start, you may have too small a pump. Ideally, you want an aquarium pump that can move 5 times the aquarium's volume per hour at the surface level offset between your sump and the aquarium. Most pumps come with charts with the pump's gallons-per-hour rating at different height offsets. Additionally, if your pump is slow, you should check the pump itself for microbubbles and blockage. You might also need to prime the pump, which consists of filling the pump and the return tube with water.
You can prevent many issues with overflow boxes through design or maintenance. Since overflow boxes often go hand in hand with DIY sump filters, you do have the option of preventing some issues at the design phase. Many overflow box designs feature baffles to divert microbubbles, or light shields to discourage the growth of algae in the U-tube. Keep in mind that the same screens that block debris and critters can clog and block the overflow, so it does take some compromise. You should inspect your U-tube daily, and check your screens and plumbing at least biweekly or whenever you see signs of trouble.