Unfortunately there isn't any substitution for regular water changes. If you're performing weekly water changes then there's no reason why the ammonia levels in your tank should be abnormal. Under gravel filters can be helpful but I seriously doubt it's filtering out the fish waste that's settling in the upper layer of your gravel so again, weekly water changes with a gravel siphon are still necessary. I also wouldn't rely on an under gravel filter alone. I'm not sure what type of fish you're housing in your 29 gallon tank but from what I've heard, under gravel filters are more of an additive to help keep your water parameters normal. However, a canister filter, internal filter, or HOB filter is still required in conjunction with your under gravel filter. Finally, do you test your aquarium water regularly? For pH, gH, ammonia, nitrates, nitrites, etc.? I'm just trying to figure out why you feel that you need the ammonia filter cartridges? I realize it's been a year since you asked this question but please, feel free to reach out for any more questions that you might have. I hope you found what you needed.