Nowadays, guppies are extremely poor quality. Its hard to describe just how much worse their health is than a lot of the other fish for sale. Basically, if you buy guppies from a store, the odds are already stacked against you.
If possible, look at local for sale sites to see if anyone is selling tank (home) bred guppies. These will be much stronger.
Then there is the soft tap water. It is correct that most livebearers (like guppies) want harder water. Luckily, the water can be hardened much easier than it can be softened.
Im not sure what "water purification liquid" we are talking about here. Is it one to remove chlorine from the water? If so, this will not be what you want to use for hardening the water.
What you need is commonly referred to as GH salt and KH salt. You could buy this from stores, but since we know what chemicals they are made of, its much cheaper and quite easy to buy the salts yourself and it saves a decent sum of money in the long run.
The GH salt raises the General Hardness.
You can use Magnesium sulfate and Calcium chloride (or Calcium sulfate) powders to make your own version of this. We can assist you with the math for how much you need to add of each.
The KH salt raises the value that we refer to as "Carbonate Hardness". (Carbonate is spelled with a K in german, hence the K in KH.)
You can use a carbonate salt for this, we recommend Potassium Bicarbonate over Sodium Bicarbonate, because potassium is a good plant nutrient, while sodium is not.
If all of this makes your head spin too much, its of course totally ok to just buy some premade GH and KH salt. You can always come back to the DIY chemistry later 😊
When you alter your water to suit your guppies, you want to make sure that youre mixing the chemicals into the water thoroughly
before you put the water in the tank. So mixing the salt in while the new water is in the bucket, for example.
And you will want to use the same amount every time, so that the water is always the same hardness for your fish.
There are a lot of nice plants to choose from! I would get at least one floating plant (recommend amazon frogbit), and several stem plants, because these grow fast and will help you keep the water healthy.
Look for any plants that are in the "Easy" category from the plant nursery. Often these will have a green label or "Easy" written somewhere on them. Here is a list of all the
Easy plants from the Tropica nursery for example.
Dont be afraid to ask if you have any questions 😊