Well you're over changing the water. A 5g only needs a once weekly 100% change. With a filter you could potentially do less often, especially if it is fully cycled. Should have a reliable drops test kit for ammonia, nitrite and nitrate if you want to cycle.
However.. you absolutely cannot keep 11 fish in a 5g.. what other fish are in there.. The only other tank mates could be a shrimp, snail, or one other betta on the other side of a divider.
If it's anything other than a sponge filter (like HOB) make sure it's prefiltered so he can't get caught in it.
Photos would be helpful. I'm not really able to picture what you're describing.