Crabs are the ultimate opportunistic feeders, and as they get bigger they have more opportunity to feed on things you might not want them to. There are two general classes of Porcelain Crabs, those which host in Sea Anemones and free living ones. Avoid the free living ones. They're usually a grayish green color with a weird striped pattern. The commensal species are white with reddish spotting, and rarely leave their host which keeps them from spreading their evil. Mithrax species are too variable in behavior, too difficult to tell apart, and have too many similar appearing species in other genera which are too truly evil to bother with. Mithrax are often touted for their eating of Valonia (Green Bubble Algae), but they're hit or miss on even that. Get a Red Sea Desjardin's Tang if you want something to eat Valonia. The main reason to avoid Blue Legs and the other Hermits is similar to reason to avoid Mithrax, other similar appearing species that get large. I see no reason to avoid Scarlets, because theres a sure fire way to id them. Look for the yellow smile when the crab is facing you. They do get a bit bigger than true Blue Legs, but I haven't experienced problems with them. Trochus are the poster child for the sensitivity to salinity change in snails that Mike referred to, and the "Strombus" that stay tiny aren't Strombus at all. Conchs are genus Strombus, and they all get somewhat if not excessively large. There is a snail commonly known as a "Dove Snail", which is shaped very similarly to a Conch, but they only get 1/2" or a bit larger. Probably what whoever recommended Strombus to you actually meant. They also breed like flies. If you want some, pm your address and I'll send you some for the cost of shipping. As far as prophylactically treating fish, I'm all for it. When I ran a retail fish room every fish that came in got a 24 hour praziquantel bath to eliminate flukes, which kill way more captive fish than ich does. Additionally once they were eating prepared food, they would get my own little concoction of prazi, metronidazole, and a sulfa based antibiotic in flake/pellet food. Hyposalinity definitely works, but it's tedious, stressful on the fish, and if you don't take salinity down to 1.010 you're wasting your time and effort. If you want to prophylactically treat for ich, use copper in ionic form. Forget about chelated forms like Copper Safe. Garlic for fish is highly overrated medicinally speaking. But it is a wonderful appetite stimulant for fishes, and it was a main ingredient in my previously mentioned concoction because it masked the awful taste of praziquantel. Captive fish health is a combination of 3 factors, water quality, diet, and suitable physical surroundings, ie suitable tankmates and hiding places. Anything which gets a fish eating better is good, and garlic does just that.