I would personally sterilize it all, and start from scratch. Use some bio accelerators, like Seachem Stability, to try and speed up the cycle a little.
And try to use a plastic box, bucket, spare tank, etc as a quarantine before putting fish on your display. You can use a seeded sponge filter from your main tank. Treat the fish for everything before putting on display, even if you have no signs of disease. This will prevent or minimize things like parasites, bad bacteria, ich, etc from getting into your tank. Sterilize everything, including decoration, substrate, fish, and plants.
In the long run, preventively quarantine and sterilization will help a LOT with your tank's health. I learned this the hard way. Even if people sell you "quarantined" and disease free fish, you should still do this, as anything can happen in LFS and fish distributors.