Have you checked you water perameters?
if you ripped it all apart.. Your likely talking a beneficial bacteria cycle. and vaccing your sand should help, it's a fine talent to master

My bets on by replaceing the sponges ect your tanks going threw a bacterial bloom. Upgradeing filters never hurts, but tbh it's liekly not needed. So the weekly WC's are keeping you from building up an abundance of toxins, which is good.. but also slowing the cycle process. I'de confirm w/ a water test. BUt thats where money would be if I was a betting woman.