I try to, at the very least, every other night. When I started the tank, I was checking it every day. This way I know exactly what is going on in the tank. If the pH starts falling, I can catch it
As Jim has started backing off on testing he will continue to do so until he tests once per week or biweekly or monthly as he gets to know his tank, but until you get to know your tank (and each and every tank is different) you should be testing often. Eventually the inhabitants will tell you when something is a little off even before a test kit would, but this takes lots of time spent with your tank and it's inhabitants. My recommendation would be to remove the sea apple and go cuss out the
LFS jerk that allowed you to buy it. Sea Apples are filter feeding sea cukes and not only can they nuke your tank, they have very demanding feeding habits. They are not for new tanks or new hobbyests, there is just to much danger in keeping one of these critters if you don't know alot about them.
