Aquatics Unlimited: Articles: The Overflow Box
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The Overflow Box

for the week of 8/12/99

Also known as a siphon/prefilter or a surface skimmer, an overflow box is generally used to deliver water from an aquarium to a sump (such as a wet/dry filter) below. What seems like a rather simple concept actually results in a rather complex device, as anyone who tried to make his own might tell you.

The Parts - Water first enters the inner box, which sits inside the aquarium and usually has a serrated top edge. Any water that is higher than that edge will flow into the box. A siphon tube then draws the water up out of the inner box, over the tank side and into the outer box. The outer box contains a short standpipe, through which the water then flows into flexible tubing and then the sump.

Why the surface? - There are several good reasons that overflow boxes take water from the surface of the aquarium. First, it's a convenient to let the tank's water level determine how much water will flow through the sump. Second, there is less solid waste at the top of the tank to plug pre-filters and wet/dry filters. And finally, dissolved organic molecules tend to collect at the surface, and they may be best removed by a protein skimmer in the sump.

Balancing - It is important that both the inner and outer boxes maintain enough water depth to keep the siphon tube full of water at all times. If the siphon gets air in it, all the water from the sump will be pumped into the aquarium, and overflow onto the floor. Likewise, care must be taken that no restrictions such as clogged pre-filters, blocked serrations or kinked tubing restrict the flow from tank to sump.

Water levels - The water level in the aquarium may be set by adjusting the height of the inner box. In this way, it remains constant while evaporation and tank filling changes the water depth in the sump.

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