This is traditional shop advice - get the tank filled, make you wait a few days, tell you to buy a few products to put in your water and magically, they say your tank is ready for fish.
Basically, what happens in a fish tank that is left to stand for a few days before adding fish is . . . nothing. The water goes around, but that's about it. They make you wait partly through a misconception that having your tank set up for 3/5/7 days actually does something and partly because it makes you feel better about getting the fish, because
you this this does something.
When you actually get fish into the tank, the following happens:
They start eating and breathing. Like with all forms of life, this produces waste products. The number 1 waste product fish produce is
ammonia. This comes from their gills and from their wee and poo. It also comes from any food that doesn't get eaten and starts to rot in the tank, as well as dying plants or dead fish.
When this ammonia comes into the tank, it will just sit there. In a newly set up tank, there are no ways of getting rid of it. The shop may tell you that filter aid chemicals will get rid of it for you, or will make your filter get rid of it but this is very, very rarely true. Most bottled 'filter aid' products contain a mix of
dead bacteria, the wrong bacteria and other dead organic matter. No 'filter aid' products get consistently good reviews from knowledgeable fishkeepers. However, the shops still sell them because it's all £££.
So, how
do we get rid of the ammonia? You see, ammonia is very, very toxic and must not be present in a fish tank. The way to get rid of it is to have your filter build up a natural supply of the right kind of bacteria. These bacteria use the ammonia and form what is known as the
nitrogen cycle in your tank. However, this process of building the bacteria takes a long time - often many weeks - and in that time there is a very high risk of the fish dying or getting sick. This is more £££ for the shop as you then come back for more fish and to buy medicines and other products. Essentially, the worse a time you have has a fishkeeper, the more £££ the shop makes! Sadly, a lot of people accept that "fish die a lot" and so can have many, many fish tanks and have been keeping fish for years and years until they learn about the cycle!
The way many fishkeepers get around this problem of poisoning the fish at the beginning is to do a
fishless cycle. This is a method of using bottled ammonia or another ammonia source to grow the bacteria, so when the fish are added they already have a working 'toilet'.
However, as you have already added your fish too early as a result of bad advice, you need a different method to keep the fish safe and build up those good bacteria. This method is calld a
fish-in cycle. What you need to do to keep your fish safe is to gt a liquid test kit for
ammonia and
nitrite (another toxic component of the cycle) and test your watr twice a day - morning and night. Any time the ammonia or nitrite is above
zero, even by a tiny bit, you need to do a largewater change of at least 50%. I say at
least 50% because taking out 50% of the water takes out 50% of the ammonia. We want to take out 100% of the ammonia, or as close to it as we can! It is not uncommon to ned to do two 80% water changes back to back to remove enough ammonia and nitrite!
If you can't get a test kit right away, start doing daily 75% water changes to help the fish along.
The cloudiness is called a bacterial bloom. This happens when bacteria in the water find something they can eat - namely fish food, wee, poo and anything else organic in the tank. It happens a lot in new tanks and there isn't much you can do about it. Water changes will not help as the bacteria grow too fast. These bacteria do not directly harm you or your fish so there isn't a problem. They are different to the bacteria in the filter that help the nitrogen cycle along, but they both need each other because the bacteria that cause the bloom are the bacteria that break food and fish poo down into ammonia. A healthy tank has these bacteria, just not quite as many as are in a bacterial bloom.
So, don't worry about the cloudy water and DO get your water tested ASAP and also do some water changes.
Here is a great information link to get you going - it will answer a LOT of questions.
Btw - you DO want to add chemicals to treat the water. These chemicals remove chlorine, heavy metals and chloramine. You just don't need the 'filter aid' products.