my LFS sold me something called SERA Toxivec Immediate Protection that removes chlorine, nitrate and ammonia, he claimed if i had a teaspoon of this to my water, leave for 24hrs, it would be ready for red crabs, is this true or does the tank need cycling? he also said i'd never need it again as it lasts forever but i'm not so sure...
also whats sump mean i keep seeing it everywhere??
A sump is an auxilery tank normally hidden under the display tank used for fitration and water treatments most common on marine systems. Basically the main tank is fitted with an overflow so water pours down to the sump then along through various types of filter (foam, floss, mud, live rock) and also equipment like the water heater, skimmer and so on can be hidden down there. Then at the end of its journey through the sump it gets pumped back up to the main tank. Plenty more info if your interested.
As for the crabs, I'm not too knowledgable of crabs but this sounds like balderdash to me. Sera Toxivec is an ammonia and nitrite locking product that stops the chemicals being toxic but at the same time will prevent the filter from cycling. I'm not fully up on the exact chemical workings of the product but your store adviser seems to have forgotten that the crabs will be continually adding ammonia to the tank. Find a better LFS.