I would feed more variety, do water changes more frequently and clean the filter once a month.
Get some frozen fish foods (marine mix, prawn, brineshrimp, daphnia, bloodworms, etc) and keep them in the freezer. Feed a bit of dry food first and then give the fish some frozen food. Try to feed different foods each day. You can also give them live foods like aphids, mozzie larvae small flies and moths that have been caught without bug spray. You can buy live brineshrimp and daphnia from most pet shops and feed these once a week.
Do a 50-75% water change and gravel clean the substrate every week. If you can't do it every week, then do it every 2 weeks.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.
Clean the filter once a month. However, if the filter is less than 6 weeks old, do not clean it. Wash the filter materials/ media in a bucket of tank water and re-use them. Tip the bucket of dirty water on the garden/ lawn.
If any more fish look unwell, post pictures and a 1 minute video of them asap.