I have found that water changes in the smaller tanks-that the stir and dip method to work well. Leaving the Betta in the tank for partial water changes.
Stir and Dip method-you will need a bucket for the waste water and to use for refill, a small plastic cup, chopstick or item to use to stir the substrate.
Unplug the heater and turn off the filter-using the chopstick/wooden spoon or anything will work-Give the substrate a slight stir (avoiding the decorations
) to bring the mulm/debris up into the water column- Using the plastic cup-Dip half the water out.
Refill with like temp dechlorinated water.
Plug the heater in and monitor temp for the next hour.
Prime and restart the filter-The filter media needs a swish/rinse in the bucket of old tank water a couple of times a month.
The water should clear within the hour-If not-then you either missed a water change, overstocked or overfeeding.
You don't want or need to remove all the mulm/debris in the tank. Too clean and sterile is not healthy and its hard to support life and the balance of the system.
In a 3gal filtered tank-water changes of twice weekly 50%....1-50% water only and 1-50% to include the substrate-should maintain water quality-provided that you are not overfeeding or leaving uneaten food long term.
While I do recommend testing kits-you can still successfully keep a Betta without one-provided that you make the needed water changes. Water changes shouldn't be based on test results alone and if you make the needed water changes to start-all should be fine.
Regardless of the abnormal test results- in most cases you will do the same thing....Water change
and if you wait on a test result when you see a problem....it might be too late. Remembering that some skewed test results can be due to additives used, user error and the water itself.
As a general rule-anytime you see a questionable behavior change...make a 50% water only water change. IMO/E-one of the best water testers is the fish and the power of observation.....