The tank water smells bad can mean there is something rotting in there. How many fish do you still have left? Did you remove all dead fish immediately?
How often and how much have you been feeding?
Overfeeding can cause poor water quality and result in bad smell. You only need to feed once a day and as much as the fish can finish within 30 seconds. All leftover fish food must be removed immediately after feeding to avoid spoil the water quality. Immediate fix for your poor water quality is to do partial water change with substrate vacuuming. Make sure you the aquarium vacuum suck up all the leftover fish food and other debris such as fish poo.
What fish food have you been feeding to them?
So you are looking for a high quality premium fish food. I recommend New Life Spectrum pellets. Make sure you pick the correct size. I am using 0.5mm pellets myself because most of my fish are only 1.5 to 2.5 inch long. I have tried 1mm pellets first, they are a little too big for my fish.
For more premium fish food, check out this article.
