Ok, think have figured why your fish not doing all that well.
A once monthly water change is not all that good, and also removing ALL the water is not really recommended either.
I would suggest changing this to doing once a week water change of around 25% water changes.
When you clean the tank, you say you remove all the gravel, i would strongly suggest to leave the gravel alone, just a simple gravel vac will be sufficient.
Gravel can hold quite a lot of beneficial bacteria that is needed to process the ammonia waste your fish produces.
(be sure to read that link i gave earlier, that has lots of good information and try to ensure your tank is actually cycled)
And lastly, cleaning the filter media, just clem up to half of your filter media at a time, not all of it in one go, again, this might remove quite a lot of the beneficial bacteria that your tank and fish needs.
I would suggest, if you can, to get a water test kit, an API liquid test kit if possible, if you cannot do this, then your LFS may test your water for you if you bring a sample of your tank water to LFS, and ask for specific readings, not just 'its ok' kind of response.
Sorry if this seems kind of overwhelming but its good you were asking advice and you have an idea now of how to do things that will give your fish a much better chance of being healthy.