I would still say you have an overstocked tank because even if your upper levels aren't occupied with the loaches and catfish, they are still competing for space in the lower region, and they are still producing a lot of waste products. Luckily for them, you seem to be pretty good with water changes. I would say you should be good with whatever filter you have, but it is necessary to do water changes at least 20% biweekly. I read somewhere that it is less stressful for the fish if you do changes every week (low percent changes - maybe 5%, but to be sure of what percent you can always test your water to see what your water parameters are every week until you see a cycle for your individual tank) because your fish will then be used to your hands being in the tank as well as any foreign objects you use. Any water changes that are over 20% biweekly can be harmful to your beneficial bacteria. Also, doing only one water change a month is bad. If you haven't had any fish die off yet, I'd say you are doing well, but only if your tank has been set up for longer than 6 weeks.
If you haven't had any fish die off yet, you are probably doing a good job of removing wastes. Since you already have your tank set up, and purchased the fish, I would just suggest that you keep a close eye on the water parameters (ammonia, nitrates, and nitrites,etc. - the three I mentioned are most important). I wouldn't add anymore fish to the equation though.
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Phishie

"An ounce of prevention is worth a pound of cure."

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