Well first off I would recommend you do ammonia and nitrire tests every week,
I have alot of 3 gallon cycled tanks and they have a habit of loosing there cycles every now and again,
If you should loose that cycle you need much more frequent water changes,
You need to watch your fish for any signs of trouble, run test checks if anything seems not normal..
Otherwise for only one betta in a 3 gallon cycled tank, a water change once a week is good
Personally I'd get rid of the underground filter, Just collects garbage anyways,
Plus I have had several fish get caught up in them somehow.
Last edited by Chicklet; 04-12-2009 at 06:33 PM.