Planted Tank Enthusiast
Join Date: Feb 2013
Location: Jacksonville, Florida
A free tank (of that size especially) with no silicone + a tube of aquarium silicone + a couple hour's work is still cheaper than a new tank.
That said, I'd wager the fish don't suffer from ammonia burning so much as nitrate poisoning. Definitely a case of Old Tank syndrome that will require gentle acclimatization of the fish back to 'good' water params.