Also, the filter may be doing it's job, but that doesn't mean you don't have to do water changes - the bacteria in the filter will convert ammonia to nitrite and then nitrite to nitrate, but nothing gets rid of nitrate but water changes.
Edit: Definitely don't add an algae eater as someone suggested. The tank is barely big enough for the betta let alone another fish. And of course algae eaters produce waste as well, making the tank dirtier even faster, meaning you'd have to do even more frequent water changes.How often should I change my fishtank water?With a filtered, mature tank you should be doing weekly water changes of between 25-50%. The filter is turning ammonia and nitrites into nitrates, and leaving it at that. While not nearly as toxic as the other two, Nitrates can still be dangerous in high levels. Its up to you to take the nitrates out with water changes.How often should I change my fishtank water?If you must change it I would do 50% or less. Betta's are very sensitive to ph changes in their water. I lost a betta when I changed his food and it caused his ph to become acidic. Dispite three conditioning medications he eventually died. I felt horrible.How often should I change my fishtank water?get a couple of dwarf coryadora hill stream loach, but still change water weekly 15-20$ is plenty cleaning filter media in the water you removed from the tank.How often should I change my fishtank water?Well put a small algae eater in the tank, Then change the water once a month, it'll do the jobHow often should I change my fishtank water?i'd say about 50% every week or so
do not add an algae eater (even if ur tank is overgrown he'll eat it all in less than a week and then what?), scrub algae off if neccessary
