my sump is 158 litres. but due to it being only about two thirds full the capacity is less. I use a deltec ap600 skimmer, a deltec fluidised rowaphos reactor, a homemade calcium reactor which was added after the pic was taken. I got a glass cutter and cut all the weirs to size. your local glass shop should be able to supply them pre cut if you don't want to cut them yourself. I then siliconed them in to position and I used old books and masking tape to hold them until the silicone cured. you could use a 20 gallon for your sump. it depends what the size of the main tank is though. you will have to get the main tank drilled. if the power goes off, then the top 1-2" of the water in the main tank will drain in to the sump. for this you will need to allow space. (if you look at the pics of mine, you can see in the right hand section, the water is low and there are three white lables low medium and high. this is so I can keep the level right, if the power goes, this section floods.) i have 2x 1.5" holes through the base to drain in to the sump. and two 3500lph pumps to return the water to the main tank. as for a protein skimmer I would go for either aquamedic or deltec which are the two best makes on the market due to there impeller design.