Do you have another tank setup? I see a fair bit of advice to get some tank mates but I want to stress that you should be quarantining new fish for at least a month, 2 months would be better. I would also suggest NOT adding tank mates as you currently don't know whats wrong with your fish. Adding new fish to a potentially sick tank is a bad idea.
Do you run a heater? If so turn it up and get the temps to 78-80. If you don't have a heater I would highly suggest one. If your home temp is 72-74 your tank will be a fair bit lower and would easily be the cause for a lethargic fish.
Also, because you asked if doing weekly water changes would be enough and then mentioned that you did a 20% wc, i am led to believe that you don't do water changes that are large or frequent. So my advice would be to start doing 50% water changes even other day for a week and then up it to 50% daily. make sure the water is about 1 degree warmer from the tap then the tank. If after the 2 weeks of increased water changes prove to not be helping then you can assume your water is not the issue, this is also assuming your tap water has been tested and is of good quality for your fish.
Other suggestions I have to go along with the water changes and increase temps are to dim the lights or block some of it somehow. It could be that the bright lighting is stressing him out. Like suggested adding caves or pots and some decore, fake or live plants, some driftwood would be all very good.
Look at floating plants like frogbit or water lettus, good for helping with water quality and blocking light.
This is all I got for now, if you do this stuff and no results I would imagine its a disease issues and may have to look at medicating. But its best to explore the environmental issues first as meds can further stress and we still don't see visual signs of illness.
Hope this helps. keep us posted.