A sump is a tank under the main showtank.It is connected to the main tank via some sort of overflow and returned to the main tank by a return pump.A sump can be used both on marine and freshwater systems.It provides more water volume,which aids water stability, and can be used as a means of filtration too. There are two ways of feeding the sump from the main tank.Both are basically an overflow. one way is to have the tank drilled. Another way is to have an overflow box fitted, this is used for undrilled tanks. the over flowbox provides a means for the water to overflow into the sump. The water overflows from the tank, into the box and down the pipework into the sump.The water is returned to the main tank from the sump via a large pump. the sump can have a deep sand bed and Caulerpa growing on the sand, The Calerpa basically feeds on the nutrients in the system thus removing them, the deep sand bed will denitrify, the sump is a good place for the skimmer to and also to hide the heaters in thus getting the equipment out of the main tank,
Sumps as decribe above using cualerpa can also be grown using miracle mud or simular products creating a a nutrient rich bed for the cualerpa to grow in. This method is often called several names including Leng Sy, eco system and algae scrubbers. I'm just in the process of building one for my new 915 litre system.
beware i have a sump trickle filter they are great but make sure if you have a power cut and the pump stops working that the tank underneath doesnt over flow,check by simulating a power cut and watch the tank below.
if you drill a hole in the return pipe just below the surface of the water it'll cut out any syphon action from that as well, your sump shouldn't be more than 4/5ths full when the tank is full up to the bottom of the weir/overflow to prevent any flooding risk
There are no experts, mearly beginners who know more than others.
Support your LFS, online sites won't stay open 5 mins extra for you