Why let a tank cycle?

[bluespotben]

I work selling koi in the uk. When a pond and filter system is set up there is no bacteriea present in the water or filter media until fish are added. this as i understand is because fish waste breaks down into ammonia then nitrite and thenm nitrate and so on, so if this is the case fish have to be present to start of a biological filter. so why do you have to let a marine tank cycle for some weeks with no fish present? shorely there will be no bacteria present to be able to start up a biological filter and this practise would be a waste of time? any explanations would be greatly appreciated! Thanks guys and gals !

[Ninong]

Many local fish shops still advise new hobbyists to purchase a few hardy marine fish, such as damsels, to start the cycle in their saltwater tanks. It's really unnecessary and cruel to the fish to cycle a tank this way. If you want to cycle a marine tank without live rock, you can do it with a few dead shrimp from the supermarket or even a tiny amount of ammonia from a bottle.

Reeftanks are a different story entirely. Fish are much more tolerant of nitrates than invertebrates, such as corals. So it is very important that a reeftank be fully cycled before any corals are added. In fact, even some fish (angels, for example) should not be added until the tank has had a chance to settle down and mature for a few months.

[bluespotben]

So how do i cycle a tank with live rock or sand then, im just setin up a 4 foot tank and have 1 blue atinic light and 3 life glo bulbs. i have a red sea prism protein skimmer and an ehiem professional external filter. i also have a 15w uv. wat would be the best way to set up a fish only tank abd how would i cycle it? thanks again

[Ninong]

If you are going to use live rock and sand in your marine tank, just put the sand in the tank, fill it with saltwater and add the live rock. The natural die-off from the live rock will provide the ammonia to nourish a growing population of bacteria. If you test your tank water, you will notice the ammonia readings rise and then fall while the nitrite readings continue to go up. Eventually the nitrite readings will fall while the nitrate readings rise. Within a matter of two to four weeks you should have zero ammonia and zero nitrites. Assuming that your nitrates have also fallen to tolerable levels, it is now safe to add livestock to the tank.