rr or r/r---'reef-ready': means that a tank is drilled to accept downflow and return hoses, and that it has a downflow box surrounding these bulkhead fittings. A proper internal downflow box has a 'weir' or grating of teeth to prevent stuff floating over the edge, and a 'double wall', which prevents water from draining from the tank any lower than the bottom of the top weir, even if there is a secondary weir near the bottom. Into the downflow box go pipes that connect to the sump hoses via the bulkhead fittings on the tank itself. Important: whether the rr rig is factory-installed or diy.
diy---as in the rest of the world, means 'do it yourself'. A diy skimmer would be a skimmer created from scratch or 'modded' by a reefer.
mod---'modified'. Or as a noun, a 'mod'. Or verb: 'modding'. To alter a pump or other device for better or more specific performance.
mj or MJ---'Maxijet': a brand of pump. mj's are often 'modded.'
HOB or hob---'hang on back'. A device, often a skimmer or downflow box, hung on the back of the tank and connected by U-tubes. A sump can be connected to a non-reef-ready tank by means of a hob downflow. A Remora is an example of a hob skimmer. There are also hob refugia, etc.
U-tube---a hard tube shaped like a U that is used to link two tanks or a tank and a device. It is started by the user sucking on an airline tubing at the apex of the U to draw water up and over the U. Then the airline is withdrawn and the U-tube will continue to draw until the water level falls below the source-side bottom of the tube. U-tubes are prone to clogs and low water stoppages. Another method of connecting to another tank or device is a 'bulkhead', a piercing of the tank wall and insertion of a 'bulkhead fitting,' which enables things to be screwed on or connected on either side of the opening. Skimmer outflows and external pumps are examples of things usually connected to a sump by bulkhead fittings.
sump---a 'second tank' divided by 'baffles', or panes of glass/acrylic that create a flow pattern of high-low, high low. Water flows by gravity from the 'downflow box' at tank level, to the 'sump inflow chamber', then goes to the next chamber, and the next, and the last, where the 'return pump' drives the water uphill again to the return pipe, which jets it into the display tank. Common things in the sump are: 1. the skimmer and skimmer pump. 2. the heater 3. a refugium, complete with light and macroalgae such as cheatomorpha. 4. the return pump. The sump adds water volume and provides a place for additives to mix more thoroughly before going to the display tank. Its immersed pumps may also contribute quite a bit of heat.