Yes, you would have to put all the fish in one tank. I think that the petco sale ends tomorrow, at least here it does. If necessary a 20L might do if you have the space somewhere. You also can tell your parents you wil sell it on CL
when youre done with it if that will help them to help you. Or just tell them to put it in storage in case something happens again. make sure they know you arent setting up ANOTHER tank but saving the fish. Once they are in the new tank which needs nothing but some PVC
pipe for them to hide in and a heater/filter you treat them all at once with a copper based medication, like cupramine as directed.You will just leave the main tank alone for a couple weeks so the ich can die off fully and then you just put the fish back in the main tank and youre done. Make sure you dont add any sand or gravel to the qt it will make it easier to clean and do water changes on since it wont be cycled.
I personally wouldnt put your tank water in the qt as there is most likely free floating ich in it.
You can also use hyposlainity treatment on the fish in the qt, whereby you lower the salinity every day down to a low number. It helps the ich fall off the fish and lets them breathe easier. And yeah, you'll have to do water changes as needed based on the water tests. It owuld be a good time to learn not to overfeed your fish as well.
They dont actually have to be fed everyday, you can skip a day here and there. Unless you have special fish that need to be fed daily.
Looks like you might be able to do this, I;m glad and your fish will apppreciate it too. Glad your parents like the fish gives them incentive to help you.
Please let us know how you are doing, and don't add anymore fish until this is all over with.