Hi Anthea,
Sorry about your losses.
You could try mixing some fresh garlic with your food. Garlic stimulates their appetite and boosts their immune system. Make sure you syphon up all uneaten food. Also you pH of your tap water straight from the tap will be lower than what it would be sitting for a day. You may have soft water, which is more prone to pH swings. I sit my water for at least a day and then an hour or so before w/c plug in my pump and heater. This brings the water to it's true pH, so there isn't such a swing to stress the fish during water changes. Is your tank second hand btw? If so you could have inherited disease from the previous fish kept in there.
Hope things start looking up from now on. Good luck!!