Reverse the lamps. Put the ones on the right on the left and vice versa. See if the problem moves with the lights. I'm betting that we've found your issue. If these are PC lamps, then 8 months is past the usuable life and they need to be replaced. If they are T5s, you either have crap factory lamps, or it's just the imbalance in ages (generally you want to replace all lamps at once) that is causing the outbreak to favor one side.
Also, did you simply remove the HOB filters when you put the canister in place? If so, it could have disrupted your nitrogen cycle and caused ... well ... anything's possible... to happen. You generally want to phase in a new filter over time. If that's what you did, then I'd just wait the situation out.
Be sure that filter doesn't have gunk building up in it. Canister filters require regular cleaning lest they become nitrate factories that cause all sorts of problems... just like this.