It can be hard to measure a betta exactly. In most cases, it will be enough to simply get an estimate by holding up a measuring tape against the glass. Be patient since most healthy bettas do not stay still often.
Make sure you know the sex of your betta. Female bettas never get long, flowing fins like the males do. Don’t confuse normal wear with injuries from diseases. Crowntails naturally have ’torn’- looking fins.
Also, while a healthy adult betta will swim around vigorously in its tank, old bettas will hide behind plants and décor and swim sluggishly. When feeding your betta, watch it to see how quickly it spots its food. Old bettas may swim slowly to their food and miss it a couple of times before spotting it.
