You fry who disappeared probably died already of Ich. They may be more vulnerable than the larger fish and go first. Their bodies are so small or easily eaten (thus spreading the disease further) that they aren't always found. By now all of the fish have been exposed to it.

You wish to keep the number of Ich organisms in the water and on the fish's bodies low = at a non-lethal level. That means you need to take action on the following today. If you don't, their numbers will increase, the immune systems of most or all of your fish will crumble and they mostly will all die.

In terms of treatment, your shop is only offering part of the treatment. They didn't mention doing a partial water change of perhaps also 25%. Neither did the mention stabilizing the water temperature and then walking it up, maybe a degree every 12 hours until the tank is about 80 degrees F/ 27C. Lastly they mentioned nothing about a treatment. Call them and a competitor of theirs for suggested medication(s). If they have nothing to offer, look for a competent fish dealer.

As for the salt, once you have added a certain amount of salt, you don't want to just keep adding salt without changing the water! To do that would be to possibly poison the fish with too much salt!

If you added one teaspoon per gallon, add no more until you change water. If you change out and add three gallons of water, add three teaspoons of aquarium, not table, salt.

There are some excellent explanations of when the Ich organisms will be vulnerable to treatment in the Immediate Help section. Also, appropriate meds are suggested.

You may have already cost yourself and your fish valuable time by not looking there.