Desease In Tank

I had a tank of gourami, with an UnderGravel Filter and a sponge filter on.
In it I had a mix of pairs of dwarf gouramis. As my final pair I bought some
honey dwarf gouramis. when selecting the pair, I noticed that one of the
fish in the tank had, what I thought, was a cloudy eye. I asked the fish
store guy but he said no its fine, so foolishly I bought a pair anyway. I
plopped them in the tank with the other fish (no I didn't quarantine them
=( ).

About a week later all my fish started dieing. There were no outward signs
of anything wrong on most of them, one or two had a funny eye, the same as
the fish in the store. I performed water changes and finally was left with
three fish which seemed to have weathered the storm. However yesterday
another one of my three died, he appeared fine the day before, but when I
look the next day he was dead.

It could have been old age, because there was no outward sign of illness on
him and its been about two months since I had the problem, but I'm not sure
it was just old age. I would like to restock the tank, but I was wondering
if I should empty it and scrub everything down, with either bleach or
boiling water and start again from scratch?

or would I be safe to just restock the tank?

Stu