That helps some... can you tell me if all of these fish are platys? If not, what other species of fish are in there and how many of those?
We are going to need a new set of water tests and exact numbers to work with here. Considering your description of the tank thus far, it would not surprise me to find there is a water quality issue going on. Please ask the store to write down what the exact number results are for ammonia, nitrite, nitrate, and pH, and also what type of test kits they are using. If they are using test strips it is a waste of time to have them test it because strip tests are very inaccurate and seldom performed properly.
I am going to strongly suggest you obtain your own liquid test kits
as soon as possible. API sells a master kit that is not very expensive, and it can be purchased in various places online. Right now Fosters & Smith has it for 23.99 and it includes all of the tests you will need. You can find it on their site here:
Aquarium Water Testing: Aquarium Pharmaceuticals Freshwater Master Test Kit
There are 2 things I can suggest for you at present before I have test result numbers, and both should help. Increase your water changes to 25% each week and slow your feedings to 1/2 the amount you give them now and only every other day. I promise they will not starve. Platys are omnivore, but known to be a wonderful algae eating fish. If you are finding a need to scrape algae in the tank that is a natural food source for them. If they are hungry beyond what you feed them they have the ability to find natural food in the tank at any time.
A few more questions for you, so please bear with me...
How often are you scraping algae?
How much water do you change each time currently?
How big are the fish?
Can you get any clear photos of them and their tank?
There may be more questions as we sort this out, but we're making progress. Keep the info coming...