So the cloudiness only started happening after the dead leaves rotted in the tank?
If you are sure you have 0 ammonia, 0 nitrites and 15 nitrates and fish act fine then I think everything will be fine.
Even though you overfilter (good thing) the beneficial bacteria colony is only as big as the current bioload. So maybe the rotting leaves just caused a mini cycle until your BB get it in check. The brown algae is most likely diatoms.
Doing water changes while this is happening may be prolonging the cloudiness (water changes would remove the microscopic bacteria that make up the cloud and so the filter would never get to adjust, and the cloud would just keep reproducing since it outgrows everything, just needs a little time to overcome), like i said if all levels check out safe, I would just leave the tank alone to adjust. Only do a water change if it gets really murky, and most likely the toxic levels would be in the harmful range.
I doubt the plant really caused problem, I mean not by itself, it may be your conditions for the plant werent adequate and that caused it to decay and pollute the water.
Although I never had a minicycle/bloom from such a small bioload increase. Maybe the water changes didn't fully get conditioned yet (water conditioner to remove chlorine, etc) before running through your filters and thus killed off some beneficial bacteria causing a minicycle. But it seems unlikely since you've been maintaining them for over a year.
I have heard of artificial ornaments causing cloudiness from whatever chemicals on them. Also your substrate (gravel) would have nitrate pockets build up underneath and if big enough pockets of the nitrogen gas are released it can cause a minicycle real fast.
Last edited by WaterLife; 07-08-2015 at 05:32 AM.
Reason: added more info