It would appear that you have a 75g (US) tank. Truthfully, 6 months is not old by any means for a sw tank, especially one containing an anemone. I hope that your specific gravity is more along the lines of 1.021 - 1.022, which would be an ok range to be in. However, many of us keep our reef tanks closer to 1.024 - 1.026. Regardless, that's a lot of fish to add into a marine tank all at once. Unfortunately, the sudden addition of that many fish probably caused an ammonia spike, which is a possible reason for some of your fish to look poor and / or die.
How much live rock do you have in the tank?
Do you have any supplemental filtration (protein skimmer, canister filter, wet/dry sump, etc.)?
If you don't have test kits, you should see if the local fish store near you will test your tank water for you. Alkalinity, pH, ammonia, nitrite, nitrate, phosphate, and calcium are pretty much the standard parameters that most of us have test kits for, with some also testing for magnesium as well.
I doubt that flake food is killing your fish. If you're overfeeding them flake food and there's a lot of excess, uneaten food in the tank, that could also be creating water quality problems resulting in poor fish health.
Do you have any sort of clean-up crew in your tank (snails, hermit crabs)?
A 50% water change can be quite a shock for any tank, especially a marine tank considering how sensitive some inhabitants are. If the temp, pH, specific gravity, etc. of the new water going into the tank are not right on with that of the water taken out of the tank, a 50% water change can provide a good shock to the system. I would recommend performing slightly smaller water changes a little more frequently, say 10% weekly or 20% every 2 weeks, and save the 50% water changes for emergency situations where it might be more necessary.