Unfortunately you are very overstocked and the fish you have are HUGE waste producers, you may very well have a high amonia level in the tank which is toxic to the fish. How long has this tank been set up? Can you list your amonia, nitrite and nitrate?
read this again. it is important.
each of your goldfish will reach over a foot long. your fancy may only get to 8, but that is beside the point. i assume that you picked up some little, cute, 2 inch long common plecos. if so, bring them back. the commons get almost 2 feet long when full grown.
goldfish are temperate/cold water fish. they like it around 70 or so. plecos need their temps to be around 78 or 80. they are not compatible species at all, and both are ridiculously messy. if you want a pleco to "clean up", forget it. it may eat your algae (and since you just set your tank up, i doubt you have much) but it is a poop machine.
my advice is to pick out ONE goldfish you want to keep. bring everybody else back to the store and trade for liquid test kits, NOT the little dipstick ones. you need kits for ammonia, nitrite, nitrate, and ph.
when you get home, test your water. we want to know what the levels are, so post them, just for fun.
you want ammonia and nitrites to both be under .25. nitrates should be between 5-20 ppm, but focus first on the others. if your readings are higher than .25, you need to do water changes. 50% changes, use dechlor, match the added water to tank water temp. if you do one and the levels aren't down, do another. keep testing. since you have gotten rid of the excess fish by this point, you don't have to be a testing nazi. check it daily, if levels are up again, change water as necessary. one goldfish should keep the cycle going at a reasonable rate.
if your chemical levels are under .25 without doing any water changes, then we can assume that your tank is cycled already. great. slowly, you can add other compatible fish. an oto would be a good algae eater for your tank size if you really want one. you could even get three. just remember that they don't live off algae alone, they will need some kind of sinking pellet or tablet food. plecos aren't a good option.
