I've found the julii's to be extra sensitive to poor gravel cleaning compared to other cories. the washed out then dead effect sounds like an ammonia issue or poor acclimation methods. and filter clean out is a bit excessive (only when flowrate drops)
I would get some water tests done asap (also check out your nitrate levels as even though its not as toxic as NH3/NH4 and NO2, high enough concentrations do cause problems (sometimes quite unusual ones)) 25% fortnightly PWC sounds pretty good compared to some although imo I would up it to weekly changes.
what dimensions is your tank?? if its 220L tall it could be an oxygen issue.
Check current flow as well, some of your fish prefer faster water movement, some prefer slow
what sizes are your fish, as imo the tank is overstocked for adult sizes, and also I would consider removing the chinese algae eater as they get nasty when they get older