i am not knowlegdable enough about bettas but in a 10 or more gallon I would have a filter. A filter that has a control of min or max filter and turbulence of water, the less turbulence the better. I have mine in a 3 gallonb and I do a 100% change every week because of no filter, but you do not have to do that in a 10 or 30 gallon, a waste of water if you have a filter. That would be alot of work, if you have enough of time and patience of taking 2 or more bettas out of tank and change al the water every week. Some people have a 30 gallon split for many bettas, they have a filter so they don't have to change all the water every week. Over 5 gallons I think it is smart to have a filter.