A protein skimmer or "foam fractionator" is a device used in saltwater aquaria to remove organic compounds from the water before they break down into ammonia and nitrates. It all depends on the amount of bioload in the water and how much you overfeed your "livestock" Figure out the water depth, then get your cubic inches and convert to gallons.
4" deep x 48" x 96" gives you 160 gallons, double that and you'll need a large skimmer rated for 320 gallons! or three of these:http://www.aquacave.com/es-100-venturi-protein-skimmerbr-by-precision-marine-506.html
running day and night.
6" deep and you'll be jumping to 5 of these to manage 478 gallons!
Speaking SOLELY from from my experiences of running a salt reef, you want to look at you gallons you can cycle thru a skimmer per hour versus the overall flow of your entire system. A healthy water column needs to have at least 10-15% of your total gallons per hour skimmed. For easy consumer choices, your skimmer companies nowdays rate them for gallon capacity. As long as you keep the water moving it'll skim it. I bought one similar to the Reef Octopus on this page:http://www.saltysupply.com/Protein-Skimmers-s/24.htm
for my 55 reef.
Once I understood the theory, built one just like the Aquamaxx on the same site.
When it was all said and done, I needed 2 Reef Octo's for my 110 gallon salt reef. I had an ENOURMOUS amount of live rock in it also, which helped some, but not a lot.
Most of your waste comes not from the crab, but what you feed the crabs. Once the food/waste gets in the water column, it eventually degrades into nitrates which can't be removed unless you do 50% water changes. If you use a protien skimmer it removes a LOT of the waste particulate matter in the water column, increasing the time between water changes.
Most of your fish stores that carry salt water fish will be able to hook you up. I set mine up in my sump under the tank and it was very efficient. Adjusting the airflow carefully so you remove JUST the "slime" that flows into the collection cup. Too much air and you seperate too much out of the water and your cup will overflow daily. Too little air and your bubbles wont carry the nasty stuff to the top. Once you get the air right you'll know it, take a good whiff! LOL
Make it simple and easy. I'd go with a rubber feed trough with a flat bottom filled with "bioballs" which can snap together to make a huge block so they wont float loose. Get as much as you can in the sump and cover it from sunlight. This will create a healthy habitat for the good bacteria in your sump. http://www.amazon.com/Wet-Filter-Replacement-Balls-Each/dp/B000N38DAS/ref=pd_sbs_hg_5
When you look at how stuff decomposes, it breaks down aerobically and non-aerobically. (with or without oxygen)
Non-aerobic happens in a deep sandbed, once established, the good bacteria needs to be left alone to do its job.
Aerobic is easier to do with filters & bio balls and such. Most of your decomposition will have to be aerobically since you wont have a sandbed.
To kick start the system bacteriologically, get it going with a bag of reef grunge from www.GARF.org
. That'll get your bioballs filled with the good ammonia-feeding bacteria. Keep the temp and salinity levels stable and you can get a great system running and a year-long supply of softies! You mentioned this will be in your garage? I'd look into an apartment fridge with a bucket of cold water in it and pump some of your water through plastic tubing colied in the bucket to chill your water a bit. thermostat relay hooked up to the water tank would switch it automatically. Worked great for me in San Antonio for my mushroom tank... (I fed them LOTS of iodine and needed a seperate system for growing shrooms...)
Regardless, you first stop will need to be a fish store. Ask to see their filtration system in the back and let me know how much they spent on ozone generators...
I forgot the exact numbers on ppm and this and that, but anyone selling salt life can school you proper on the numbers a healthy system should have.
Once you get it running post some pics!! then email me and I'll send you my address... LOL