HI, WELCOME TO FISHLORE!
Often clerks at fish stores don't have a tremendous amount of experience in maintaining healthy home aquariums...I say this with all respect, and am not trying to give you problems, rather I am only wanting you to have a healthy tank.
If you took water into a store from a tank that never had fish in it they could not test it and find ANYTHING wrong....of course, there were no fish in the tank to start accumulating waste, ammonia, nitrite or nitrate.....When you read the link provided you will see that it is a long process your tank has to go through if you just stick fish in an uncycled tank!
Most here recommend doing the nitrogen cycle with NO fish in the tank, but "feeding" it with either pure ammonia, or fish food, or a raw shrimp or piece of raw fish (these last two are a really stinky way to go).....This NO fish cycling takes about 2 weeks. With fish takes 6-8 weeks with DAILY water changes to keep the fish healthy.
Despite what you have been told at your LFS, the fish will die if you don't have them in a cycled tank, unless you are careful to do the water changes. The ONLY way you can be sure yourself if your tank is cycled is to buy a test kit and test your own water. We recommend the API Master test kit (liquid), it gives accurate results if you follow the directions.
The problem with relying on your fish store to test your water is that they often don't give you the test results, they will say your water is "OK", but with out having the results you won't know for sure.....I suggest testing every day right before your water change and charting the results, it will help you understand better what is going on in your tank.
When the cycle starts the ammonia goes up....until bacteria grows that will convert this to nitrite. BOTH ammonia and nitrite are toxic and will kill your fish. Water changes at this time is the only thing that will save your fish. Adding Prime will help, but cannot be relied on to bring the levels down without water changes. Then after several weeks, your nitrates will start coming up....once again it takes time for the bacteria to develop that will do this conversion. Once your ammonia and nitrite both go back down to 0 and your nitrate is up around 10-20 your tank is cycled and safe for fish.....At this point you can back off to weekly 25-50% water changes, but remember to test your water before your change, as it is the only way for sure for you to know how much needs changing....You have to maintain your tank under 20 nitrates to keep healthy fish.
As for the Pleco.....here on fishlore we ALWAYS recommend you counting your fish as if they ARE adults when first stocking your tank. It takes time for the bacteria to build up enough to handle all the waste the fish produce, and as your fish grow so does the bacteria culture.....1" of adult fish per gallon is the stocking rule of thumb...so many people buy baby fish then are never ready in time to move them into a proper size tank it is easier not to over stock to begin with.
IF your pleco is a common pleco it WILL grow to 18" They produce long strings of poo that will wrap around your tank, LONG before you are ready to concede it is a problem....they eat lots, and make a huge mess.....I love plecos, but will never again have a common just for this reason....Bristlenose plecos are a much better choice as they are great algae eaters and only grow to 5".
People here at fishlore are only trying to help you out of the same problems we have heard a thousand times....it is so much easier to cycle fishless, and not overcrowd. Good Luck!
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Welcome to FishLore. Kate have given you great advice. One thing I will add, your tank looks over crowded.
