[Olympia]

Alright, I am not going to read everyone's comments so sorry if this is repeating.
1mg/L is equal to 1ppm if we haven't established that. Any amount of ammonia results in permanent damage, though fish can usually recover -slightly- and live out their life.

I would just do a 100% change right now.. At .5mg/L you are still at .5ppm, too much.
Some time in clean water should let him recover a bit.
Don't worry about feeding, he's stressed and doesn't need food at the moment. Give him a 1-2 day fast.
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[Olympia]

Don't worry about BB.. your 60% weekly changes should be enough. Or whatever keeps the ammonia down enough! Is your filter cartridge really dirty looking? Rinse it out in old tank water.
Cleaning really shouldn't do anymore damage than leaving him as is. :-D