You could try useing a small corner filter that uses an air pump. I use them in my betta tanks because I can control how much carbon and filter floss to use. Without a filter I notice that the water has a cloudy tint to it just 24 hours later, tap water just doesnt look "clean" know what i mean? I had a corner filter in my community tank that wasnt doing much more than just adding bubbles to the tank, so i took that and dropped it in one of my betta tanks and within hours the water was crystal clear and stayed that way much much longer even after I took the filter out and dropped it into the next betta tank. So i went out and bought two smaller ones and just used a plastic splitter off the air pump so that the water in both tanks isnt too rough. With a small filter you shouldnt need to do 100% water changes often, or at all really. I do partial water changes most of the time, if theres a serious reason, i'll drain the tank completely but I honestly think its more stressful to the fish to do this all the time, i know a lot of people do full water changes but I really dont like removing a fish, catching him, whatever you wanna call it. When i first got my bettas you could tell they get nervous when you start cleaning and taking water out of the tank, probably cause they always knew that soon enough they'd be netted and taken out while the tank was cleaned, now that my boys are never taken out of their tanks they could care less about me pokeing about in the tank. I also took a clear party cup (plastic cup) and added a little gravel to it and then attached some java moss to it, so made a little planter basically and what i found is when i start cleaning the tank the betta will just swim into the cup and wait it out, so i can, if i have to, take all the water out of the tank and not remove the fish since he's lazyly swimming in the cup of warm water with an umbrella of soft plant above him. as the water gets replace and the level rises he just waits till its up to the level of the cup and then just swims back out, no harm done and very little stress (if any).
another thought is java moss makes a great bed for the fish at night, mine sleep on it every night without fail and it helps keep the water clean, and pulls its nutrients from the water and waste, not to mention adds oxygen, plus like i said the planter seems to be fun to play in so its a win win situation. I dont know if you can find some java moss but I got a palm sized bundle for just 2 bucks and am currently growing some in a glass vase as well so I can add more to my betta tanks or community tank.
hope that all helps, sorry for the long answer!
