I've seen on large sump systems at a pet store, that there is a line for minimum and maximum water levels. Out of curiosity what happens if the water goes below or above those lines?

There is usually some give to those points on the sump. Below means your evaporating, and you don't want the return pump out of the water, if its submerged. Above it and you stand the risk of overflowing if you have a power loss, when some of the water syphons back into the sump from the tank and lines.

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If it goes below your return pump can break suction and either produce microbubbles, run dry on is some situations lose prime and burn up.

If it goes above the line you run the risk of overflowing the sump if power goes off and the display(s) siphon back to the sump. you always maintain enough room in the sump to contain a backflow in an emergency.

Other related things can be skimmers perfrom best at certain levels or ranges so you want to maintain that level or you may have reactors, heaters or other equipment that needs to remain submerged.

Just keeping it simple in my way, basically you can get floods.


If your overflow fails water in the upper container (usually the display) rises and unless the sump runs out of water it floods. So the sump upper line is to make sure you don't add too much water when toping off for instance.

also under power outage some water drains to the sump and can flood the sump. Again the upper line should be calibrated to prevent the sump flood.


Lower line is more of a reminder to not run the pump dry and to know when to top off.


my .02

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fw leiden since 1979, fo salt since 1979, mixed reef 55g 2002-2009. Strong emphasis on the tank taking care of itself. Balanced with plant life, no water changes, tap water, no filters in FW. Only dosing calcium, alk, mag in marine reef tanks. http://www.aquariumforum.com/f15/my-...ods-26410.html
recent tanks (till 2009) 7 years- 10g FW leiden 7 yrs, 55g mixed reef 7, 2 yrs, 20g FW leiden, 10 g fw leiden , 29g mixed reef, current tank 55g leiden