Doing 50% changes twice a week. I have the xp's stuffed with foam on the entry and exit, and loaded with ceramics in the middle; the AC is loaded with just the foam and it gets cleaned at each water change. The XP's get cleaned onec a month, but piggy backed so one of them is cleaned out every 2 weeks. Always filled with detritus. Nitrates arent so much an issue as the constant particles in the water. Through the entirety of the tank's life since being set up back in January, there has always been a slight haze to the tank. If I sit at one end, long ways, and look through it, it's murky and not clear at all.
as for tank upgrading, have a 300g on hand that i picked up after setting up the 125, but no where to put it at this time. Trying to manage what can be done until I'm able to clear wall space to move the 125.