need advice on a filter/skimmer

so i have had a 20g tank with a 13 gallon refugium running with about 35 pounds of live rock. it has been setup for over 6 months just running with nothing in it. I didn't have the money to buy fish/snails/crabs, so i just let it run. I have finally invested the money in fish. I ordered them online and they are to arrive this wednesday(20th). Problem is i just realized that i have NO skimmer or filter. The tank will have no corals and will not be heavy stocked. Does anyone know a good filter or skimmer that i can pick that will do a good job? There will only be 3 fish in the tank with 10 crabs and 10 snails. Please post if you can. thanks.
 
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