Star daddy. You really really need to buy your own API master test kit. Do not trust pets at home. Do you remove the bodies of the dead fish? If not that's probably why your tank is smelling. You also need to clean the tank way more than you are.
If you have taken all your decorations out and scrubbed them in tap water then you will have lost your beneficial bacteria that was on them. Do you clean the filter sponges? If so then how? If you clean them in Tap water you will keep destroying your bacteria that you need.
We can't get to the bottom of this without you buying your own test kit and posting the results on here. We will help you to test the water. You need to buy that liquid test kit mate.
I'm in Manchester and I know that most chain stores like pets at home are rubbish. You can order the test kit off amazon for half the price at pets at home.