How much conditioner do you use and what brand is it?
That's good you can do it every week though. See it's not just the fish poop that you're taking out but there are other non-dissolved and dissolved organic compounds in there that you take out. Also fish get their minerals from the water the same as we do so eventually all those minerals get used up and so it has to be replenished and that's where water changes come in helpful.
Also you said his fins were curling? That could be from two things; ammonia is too high, or you have hard water. Harder water tends to cause fin curling and it is not reversible but it's not dangerous for their health either, it just might not look as nice lol The flip side is it's from ammonia so if your tank isn't cycled then this could be the cause of it. Thankfully most of it is reversible, depending on the severity they'll usually uncurl a little bit at least when they start to get good clean water. They may not fully uncurl but it might get a little better down the road is all.
Also, every tank needs a cleaning once a week unless it's an NPT (natural planted tank) in which case the plants do the filtering and then you just replenish the water when it evaporates and do about a once a month cleaning for it. But regardless, regular tanks no matter the size under 100 gallons or so should be done each week. However this also depends on what's in the tank too. If you've got a 150 gallon tank with fish that poop crazy amounts, yeah that should be cleaned out once a week lol
Betta's, for their size do not have a huge bio-load but they also excrete a type of urine, not the same as we do but they do excrete it along with their poop and just constantly through their gills. So that's why it's also important to do changes
every other week will be fine if the tank is cycled and has plants and stuff but otherwise, once a week at least should be your goal. My 10 gallon has 3 Betta's, all divided but that sometimes I even do twice a week around 80% because I can and because they poop a lot!
Oh that's another question I have, how much do you feed him and what is it and how often?
(sorry for the longer post