Not sure if I should add a sump or not?

[isaac1]

My tank is w-12 1/2 l-24 1/2 h-19 not sure what the volume is my freind gave it to me said it was 30 gallons but I'm thinking it's only around 20 it's a marine land bow front It's been running for about 8 weeks now fully cycled parts include t5 ho dual lamp, red sea prizm skimmer, 2 koralia 425 gph power heads, a flu valve 205 canister filter, and a jäger fully submersible 100 watt heater, for live rock I purchased 35 pounds from big als and blue world, and a 30 pound bag of Carib sea live sand, so far I've stocked it with two small occelaris clowns one peppermint shrimp two large turbo snails and one hermit crab( all purchased from blue world) also a couple crabs pretty sure thy might be emerald or gorrila( came off live rock purchased from big als) now I'm wondering if I should add a sump tank on my floor or if I should just keep it the way it Is and do 5% water changes weekly? All advice is appreciated!!!