try getting some prime handy, it is good at neutralizing ammonia, it could help you out between water-changes, if you don't have much circulation try getting some bubbelers in the tank (this will help gasses transfer between water and atmosphere) also try adding a bit of aquarium salt (if it is safe for gars, im not sure) i read that it helps with ammonia somewhere here but ant remember exactly. It could be "old tank syndrome" which caused this however im not sure. I know that you are a very experienced fish keeper and this shouldn't be much of a problem for the likes of you!
hope i could be of some help, good luck
alex
hope i could be of some help, good luck
alex