It appears you have unfortunately fallen for what the LFS staff says.
Its weird but LFS staff are NOT to be reliad upon for good fishkeeping advice. You would have thought so but alas, no. All theyre interested in is to sell you their product, when they will have a break or luch and of course what time they finish!
Not ALL LFS are like this, there are few good staff or LFS that do actually care for thier livestock and dispense of good advice also.
Anyway, it does sound as if you have not cycled the tank, the real giveaway is the water is turning cloudy, this is likely to be a bacterial bloom most commonly found in new tank set up and in process of the start of cycling.
The use of bottled/distilled/RO water alone is not advisable for tropical fish tanks. Simply due to the fact the water is too pure, fish, shrimps, snails and plants etc actually need the minerals and trace elements of metals in order to grow and stay healthy.
You COULD do a 50% tap water and 50% RO/distilled water for your tank, but this is only really advisable if your tap water has terrible amounts of metals, chlorine and hardness etc.
So the first thing to do is to rehome or take back the fish back to LFS, hopefully for a refund or store credit, unlikely but possible.
2nd is to test your tap water for ammonia, nitrite and nitrate as well as for pH. If you dont have a test kti, then I'd advise you to get a fairly decent test kit, such as API Freshwater Master test Kit, not bad value for money ad more accurate than the paper strip dip kits.
Also you could take a sample of tap water to your LFS, they can test the water for you, sometimes free, sometimes for a small fee. If doing this method, be sure to ask for exact readings of all the test they carry out, not accept the just 'its fine' type of answer.
And as for gH and kH levels, have a look online for your local water authority and look for water quaility, should be several pages of a list of elements and various readings of number sin different scales. Be easiet to take a screenshot or share the page on this thread and we can see exactly what sort of levels you have for your tap water.
Thirdly, have a read of this article, it may take several times to read to begin to grasp the concept of cycling a tank and understanding how the nitrogen cycle works, but its easier than you may initially think and very much well worth doing.
Cycling Your New Fresh Water Tank: Read This First!
Fourth, you need to get some tap safe conditioner/dechlorinator, this makes the tap water safer for your fish and shrimps etc by simply detoxifying or binding harmful metals as well as chlorine and ammonia if any is present.
Fifth, while the tank is cycling, I'd advise you to research or ask advice what is more suitable for you to have in your 5.5gal tank, but you need to have the water parameters readings and hardness level, this is important as NickAU already very briefly explained guppies are hard water fish meaning they thrive in harder water than soft, and vice versa for soft water fish such as most tetras.
One other thing, a 5.5 gal US is a fairly small tank, your stocking options are severely limited to just a few choices already. If you want to, you can upgrade your tank for a larger tank, say at least a 10 gal if
guppies are what you'd like to keep to start with.
But
platies and
black skirt tetras require a larger tank around at least 15 to 20 gals US
Am sure some LFS stores do one dollar per gallon sales, so may be worth waiting for that as its very cheap at $20 US for a 20 gal US tank.
Sounds complicated, I know, for the new fishkeeper, but please do not be discouraged as its a lot to take in but this is one of the most rewarding hobby you can have when things are going right. With the right advice and research we can help you along the way
