You may clean it once a week, but doesn't mean it isn't water quality, hence the need for regular testing and any daily spot cleaning, using a turkey baster.
Water parameters on a cycled tank are usually round the following:
ammonia 0 (anything over 0 unsafe)
nitrite 0 (anything over 0 unsafe)
nitrate 10-40 (this can vary depending on if you have live plants or not)
pH 6.8-8.0 (with pH if your water is 8.0+ and you have any trace of ammonia in your tank, it becomes more toxic to your axie/s).
Ideal tank temperature: cool rather than warm ( basically below 18-20C) - they can live in temperatures down to 5C, obviously if its that low they won't eat as much. Two of ours are currently sitting at 10C but axies are still eating every 3 days.
What have you tested for in your tank? If there are any ammonia/nitrite levels above 0 then you'll need to do more frequent waterchanging than once a week. You'll probably have to test daily and if amm/nitrite are above 0, then you need to do 20=30% waterchanges.
Do not add any chemicals to your tank, other than a waterager to treat tapwater for the waterchanges.
what is the tank temperature?
Do you add any other chemicals to your tank, ie: something that clears ammonia, nitrite, lowers/raises pH etc....
Do you have an UG filter?
Gravel, even when you clean with a gravel cleaner will still have a buildup of gunk beneath it (as will river rocks), which can effect water quality.
If you have a UG filter, the gunk falls beneath the plate which in turn also effects water quality.
If they had no gill filaments when you got them and they're adults it may take longer to regenerate. On the other hand if they've lost their gill filaments completely and only have the stubs, then there is something (possibly water quality) that is causing it.
I'm not a great fan of gravel for axies, after our 4 adults have been pooing out the gravel they had ingested in their old tanks, with previous owners, for the past 15-16 months. Previous owners handfed and also said they never swallow it, always assumed his were fat and healthy.