I'll just detail this a bit further which may help you understand. As essjay correctly mentioned, this cloudiness is (probably) a bacterial bloom; the tap water can be high in organics (there is no normal way to filter out organics and municipalities don't usually worry about it), and when the water is added to an aquarium with dechlorinator, the heterotrophic bacteria species that feed off these organics will literally explode by multiplying according to the level of organics. Heterotrophic bacteria cannot synthesize their own food so they need organic material such as fish waste, dead bacteria, fish and plant matter, etc., and while some are aerobic, many are facultative anaerobes, meaning that they can survive in either the presence or absence of free oxygen. The microscopic organics in tap water feed them.
Unlike the nitrifying bacteria that take up to 8 weeks to appear and then require 12 to 32 hours (approx) to multiply, heterotrophic bacteria appear rapidly and can multiply within 15-60 minutes. Multiplication in bacteria occurs through binary division, which means that each bacterium can divide into two bacteria, and this can continue so long as food is present and conditions for the bacteria are suitable.
I often see this after cleaning the canister filters, and sometimes after a water change, though these are usually not as thick as bacterial blooms in new tanks. The safest way to handle them is to let them do their thing. Water changes sometimes help, but not always, depending upon the biology. Clarifiers should never be used with fish present. Most of these work by causing the microscopic particulate matter to congeal into larger particles that can then be more readily removed by the filter media (pads/sponge/foam especially, the fine screening). But they also clog the gills of fish and the filtering mechanisms of invertebrates. So, unsightly though it may be, leave it.
Normally the fish are not harmed by the bloom. The heterotrophs quickly reproduce by feeding on this organic matter, and this produces ammonia as a by-product, and the sudden surge in ammonia may overtake the nitrifying bacteria that need time to "catch up," especially in new tanks where the nitrifying cycle is not yet fully established. Live plants again help here, as they can assimilate and/or take up considerable quantities of ammonia faster. Note that the bacterial bloom causes the rise in ammonia, not the opposite as some may think.
Carbon will remove organics, up to as point; carbon is limited in the amount of "stuff" it can adsorb (it is adsorb, not absorb) and once it is "full" it is no longer effective in this role, and needs to be replaced. If you have live plants, I would not use carbon because the plants work at all this and the carbon will remove some of their nutrients.
Without fish the bacterial bloom should clear within a few days, maybe even hours. Each system is biologically different, something that continues throughout the life of an aquarium--uniqueness. You can use SafeStart with a few fish. Again., live plants here (especially fast growers like floating plants) make it problem-free as they take up a lot of ammonia. Add the fish slowly, as you should be fine.
Byron.
