No adding fish!
The damage that is done to your fish's gills is irreversible. It is like having a bad burn, we dont heal burns very well because of the tissue damage. Fish don't heal their gills well and will never be as healthy as they could be if you don't take the filter cycling seriously.
I've read somewhere that the gills of fish DO repair, sometimes over long periods of time.
They thicken and become less efficient under toxic substances, and then get thinner when conditions become better.
Also that Zebra Danio fry have the highest regeneration rates of body parts and even organs.