To me a sump is less about gallons than what media you have in it. For a 180 gallon tank, a 50-75 gallon sump is fine.
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You don't need anything. You could make a sump that filters the water through sport socks and uses lego for bio media and it would work. What you need is something to catch the physical waste and something to provide surface area for the nitrifying bacteria to grow on. How you do that is entirely up to you, and if your ammonia and nitrites are zero and you're happy with the water clarity then it's working fine.
I have a 180 with a 4' sump. I change about 60 gallons on a Saturday. If you wanted to make it cheaper, then go with filter socks, I use 100 micron. Pot scrubbies for bio. Cheap, cheerful and functional.
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BTW, I tried hinting at this on the last page but it may have been too subtle. This is a stump:
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This is a sump:
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