Hi and welcome to the forum.
I sorry you've had to seek advice after losing fish.
Unfortunately, your spring-clean might have done more harm than good.
In the gravel and on the surfaces of your tank there are colonies of bacteria. Their job is to help deal with the waste produced by your fish.
The fish waste produces Ammonia which is poisonous. Bacteria eat the Ammonia converting it to Nitrite. Nitrite is also harmful to fish, but not as much as Ammonia is.
Nitrite is eaten by another bacteria watch then converts that to Nitrate.
We remove Nitrate by doing frequent water changes.
If you clean too vigorously you can remove all or some of the friendly bacteria.
This means that there aren't enough bacteria to deal with the waste.
Before adding new fish you need to Cycle the aquarium.
Cycling is the name given to the process of encouraging and maintaining the bacterial colony needed to deal with the waste produced by the fish.
If your remaining fish survives you need to do a fish-in cycle which is described
here.
If your last fish doesn't survive I recommend a fishless cycle as it's far more humane, less stressful for the fish and the fishkeeper.
The fishless cycling method is described
here.
