When you take the water to be tested they'll tell you your ammonia and nitrite are zero so it's safe to get fish. But the reason they'll be zero isn't because it's safe to get fish but because there is nothing in the tank yet to make ammonia and with no ammonia there's nothing to turn into nitrite. And Nutrafin Cycle is one of those 'instant bacteria' products that I have yet to see anyone reporting that it actually worked for them.
Fish make ammonia; it's in their waste and they breathe it out. Ammonia is toxic to fish - it burns their skin and gills making it hard for them to absorb oxygen. In a mature tank there are bacteria in the filter which turn this ammonia into nitrite - which is also toxic; it binds to the fish's blood stopping oxygen binding to the blood. In a mature tank, another type of bacteria turns this nitrite into nitrate. This is only toxic at high levels and is removed by water changes.
In a brand new tank there are no bacteria in the filter and despite any claims you might have heard for Cycle, this product is unlikely to help any grow - I've read enough reports of people who have tried it and found it to be useless. The bacteria are very slow growing, and the process of growing them in the filter is called cycling. Typically they take around 6 weeks to grow enough, though it has been known to take longer.
You have 2 choices, either do a cycle with fish, or do a fishless cycle using ammonia solution.
Either get a few fish on Tuesday (and by a few I mean no more than 4 small fish for a tank that size) and follow this thread
cycling with fishOr buy a bottle of ammonia solution from Homebase/eBay/Amazon/local diy shop and follow this thread
fishless cycling Have a read of these two then you'll be able to decide which method you want to try. You'll see that the fish-in cycle is a lot of hard work. It usually requires daily water changes for several weeks to prevent the fish being poisoned. Fishless is easier but it does mean looking at an empty tank for a few weeks. At the end of the fishless cycling method, you'll see a summary of the fishless cycle I've just done which will give you an idea of what to expect.
Whichever you do you will need your own testing kit, preferably one with liquid reagents and test tubes. Strip testers are notoriously inaccurate and most do not test for ammonia. With both methods you will need to test the water twice a day and taking a sample to the shop twice a day is not practical!
Any questions, just ask
Oh, and shops don't believe in fishless cycling. They'll tell you to add fish but not bother telling you to test the water, or they'll tell you to add a bacteria product like Cycle, not bothering to tell you most of them don't work.
If you want to use a bacteria product, Tetra Safe Start seems to be the best one, but it doesn't work every time. If it has been stored incorrectly the bacteria in it will be dead.
