Hi and welcome to the forum
LOL, slap on the wrist for impulse buying, but a pat on the back for buying fishy stuff
All fish need water that is free of chlorine & chloramine. Depending on where you live and your water source will determine what chemical is in the mains water. Chlorine is more commonly used in cooler climates or where drinking water does not have to travel far or spend too long in the pipes.
Chloramine is a mixture of chlorine and ammonia and it binds together to last for a long time.
Chlorine is primarily used in Australia, chloramine is used in American, not sure about the UK.
If you use mains water you will need a dechlorinating agent (available from any petshop) to break down and chlorine or chloramine so it can come out of the water and not kill the fish. It is best to get a couple of new clean buckets that have not been used for soaps or any chemicals/ cleaning products. Use a permanent marker (felt pen) and write "FISH ONLY" on the buckets and keep the buckets somewhere they will not get contaminated or accidentally used for cleaning.
Fill your buckets with tap water and add the required amount of dechlorinator, Then aerate the bucket of water for at least a couple of minutes but preferably 30minutes or more. 24 hours is ideal if you can. Then the water can be added to the aquarium.
If you are unsure whether you have chlorine or chloramine you can contact your water corporation and ask them or check their website.
To clean the tank, use a clean soap free, chemical free sponge to wipe the inside of the glass with. Then get a gravel cleaner like the one in the following link, and use it to clean the gunk out of the substrate and remove some of the tank water. Drain out about 50% of the tank water and then replace with the clean dechlorinated water. Do water changes and gravel cleans about once a week, or if you lose any fish or there is a water quality problem (ammonia or nitrite reading).
http://www.about-goldfish.com/aquarium-cleaning.html
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The more accurate test kits are liquid kits, where you add a few drops of an indicator solution into a phial of water. This is shaken and then put against a colour chart to see what the colour in the phial matches on the chart. the chart has numbers that are used to say how much of that chemical is in the water.
The paper strip test kits are not as accurate and should be avoided if possible.
If you have other questions just ask and someone here will help you