how big protein skimmer be for 180?

no calculation, and the suggested sizes from the manufacturers are not accurate.

best thing to do is give us a budget and we can help to recommend a good one that fits the budget.
 
I would go for a little higher then your total water volume. Example: If you've a 180 tank and a 75gal sump the total water volume of roughly 255gal. I would go for a good skimmer rated for a 250 or 350 tank.
 
Buy the largest one you can afford, trust me down the road, you will be glad you did, I've been using my ER for over 4 years now, best $500 i ever spent.
 
What Anton M said ,also remember it has to fit under the stand and be able to be emptied and cleaned . Bang for the buck wise I like both ASM and octopus skimmers.
 
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