Just a quick comment - I hadn't seen this until just now. I agree with
@Jay Hemdal and
@vetteguy53081. To the OP:
EDIT - I just noticed you mentioned an Ich outbreak in the last sentence of your OP. So - as others have said it could be a combination factors - but IMHO - the garlic did not help.
1. Sorry about your fish loss - its certainly devastating when something like this happens
2. When something happens suddenly after adding a treatment/chemical/etc - and all of the sudden fish start dying - the vast likelihood is that the additive is causing a problem - and immediate water changes/carbon is likely to help.
3. Unless you added a recent fish, etc - disease would be unlikely. (Except - as Jay said - you were treating for a disease with the additives). (See EDIT ABOVE)
4. In the future - try to be sure that you have the proper test kits (I do not think API tests are any worse or better than the others).
5. Another general comment would be - not treating for disease in a display tank unless there is no other way - i.e. use a hospital tank. Additionally - best NOT to mix multiple additives at the same time.
6. There are a lot of supplements out there - that are possibly not necessary - and potentially harmful. If you do use them - its extremely important to follow directions. I'm not trying to slam the OP here - merely mentioning - if you don't know how to do something (i.e. soak food) - you can always come to the site and ask before using or doing whatever you're trying to do.
Again - best wishes with your tank - and please update when you can.