Ignore what Pets at Home said, you can't add fish in a week. Well you can, but that means you would be doing a fish-in cycle which means doing big water changes every day for weeks - and risk your new fish dying. It is much easier - and safer for the fish - to cycle the tank
before you get fish even if that does take a few weeks.
In a cycled tank there are two colonies of bacteria and the process of growing the bacteria is called cycling.
Fish excrete ammonia but this is poisonous to fish. In a cycled tank, the first of the bacteria colonies 'eat' ammonia and 'poop' nitrite. Nitrite is also poisonous to fish, but in a cycled tank, the second bacteria colony 'eats' nitrite and 'poops' nitrate. This isn't nearly as poisonous, and we keep the levels low in a cycled tank by doing weekly water changes.
But a brand new tank has virtually none of these bacteria so we have to grow a lot more of them.
You can use fish to provide the ammonia, but as I said above, this involves testing the water every day for ammonia and nitrite and doing a big water change whenever there is a reading above zero. At first ammonia will rise but there'll be no nitrite; then once the first bacteria start making nitrite that will go up. It takes several weeks to grow enough bacteria so you'd be doing big water changes just about every day for weeks.
So that the levels of ammonia and nitrite don't get too high, you can only get a few fish to start with. Once ammonia and nitrite stay at zero, you get get a few more fish, and again do water changes if either ammonia and/or nitrite test above zero. When they stay at zero, you can get a few more fish.......and so on until the tank is fully stocked. It's hard work with all those water changes, and there is a risk to the fish.
The alternative is fishless cycling. This uses ammonia from a bottle to simulate fish waste. With this method, there are no fish to come to harm so you don't need to do water changes. With this method, you add ammonia then test as in the method in the link in post #3. When certain targets have been reached, you add more ammonia. Once the cycle is finished, almost all the proposed fish can be added at the same time. [Note - not from Pets at Home as they only let you buy 6 fish at once - find another shop]
This is much easier - just testing the water and every so often adding a bit of ammonia. And no fish get harmed.
Would you suggest buying a water testing kit?
Yes. Without a test kit, fishless cycling is impossible, and with fishless cycling you won't know when to do a water change.
SlapHppy gave you a link to the American Amazon, this is our Amazon
https://www.amazon.co.uk/dp/B000255NCI/?tag=
it's on a time limited deal at the moment so you need to be quick
Liquid testers are more reliable than strips, they do more tests so work out cheaper, and they include the all important ammonia which strips don't have.