welcome to fishlore!!!!!!
first off, you have to make sure the tank is cycled....which heres a link to explain that
http://www.fishlore.com/NitrogenCycle.htm its a ton of reading, but its something every tank needs to get off to a good start and the fish to live
at this point, I would see if you can find an
API liquid master
test kit and find out where your cycle is...you want to test for
ammonia/nitrites/nitrates....ammonia/nitrites can kill fish fast and should always be 0 with your nitrates being under 20......finding that out is most important right now..and dont rely on the fish store as they already have given you some advice that is very scarry...methylene blue isnt for ich ....and if used too much, can kill fish very fast also....as well as prime is great for helping cycle the tank, it wont cycle it on its own..so trusting that store is going to be hard ....
once you have the kit, you will know where the tank stands....for now, do daily 30-40%
water change using the prime...the prime will detox any ammonia/nitrites for 24 hours until your next water change.....and turn up the temps to 83-85f on the thermometer....the higher temps will kill the ich cycle but has to be left there for 2 weeks....do 2 gravel vacs a week to clean out the gravel on the bottom as ich spores will fall into it....keep the temps as close to the 83-85f as you can coming out of the tap when you refill it....using a thermometer in the bucket will help.....adding an extra bubble rock will help as warmer water has less oxygen and will keep the fish feeling ok with the higher temps....
feed only one time a day, just enough to let them get their fill in 2-3 minutes..usually a good pinch of food will do..the white on the bottom could be left over food decaying
I know thats a ton of information to have thrown at you at once, but its important to help you be able to enjoy the fish and keep them safe...