The food you are offering seem to be good quality and variety. Feeding will have to be gauged by you based on your fishes' ability to consume the food offered without leftovers sinking to the bottom or ending up in the filters. Thaw out the food, then offer a portion of it so that your fish can immediately eat everything, then offer another portion, etc., until they have completed the meal without food ever becoming waste. Many hobbyists dump in the whole meal at once resulting in food ending up in the substrate or taken into filters. As, I said, offer 2 meals a day.
Looks like your angel is developing an eye infection. You are right, garlic needs to be freshly minced, soaked in food and immediately served if it is to provide medicinal benefit. I would suggest offering one meal with the supplement I mentioned above, and one meal with the fresh garlic soaked food. The garlic is needed until the emperor's eye problem clears up and then you can discontinue that if you like.
Also, right now, I would suggest that you implement a water change every day. Just 2-3 gals in addition to the freshwater top-off you usually have to do. The water change is to address the eye bacterial infection, along with the food and supplement modifications. You want to implement this water change with minimum stress to the fish during the process. Hopefully, this will work for the angel. My thought is that the vacuuming that you are doing of waste in dead spots is likely stirring up some decayed organics which is ending up in the water column, degrading the quality of the water. While its not apparent in test readings, obviously any type of pollution can end up effecting fish and emperors are fairly susceptible to environmental issues.
Is this fish experiencing aggression at all?
Does your tank have live rock?
If the eye infection gets worse, then the fish would require treatment in a quarantine tank. Do you have one?
