sounds like you have a 3 chamber sump. I don't think there is an area for mud as much as there is possibly an area for a refugium. One chamber is for a skimmer and one is for a, return pump. most likely if you bought it from a store, it has a drain area, and skimmer area, and a return pump area. If it is acrylic, it can be drilled for an external return pump and you can use one of the chambers for a refugium(Live sand/Live rubble/ Macro algea w/ a light over)
30L w/10gallon sump 4xt5ho; Up and running:125g long, 30g sump, 50g fuge.
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