I stripped down my 15-gallon tank last night, taking out all the substrate ready for new stuff. I noticed a ~6 inch sq. area which was pretty dark and a bit ponky - I assume this was an anaerobic area. I had a similar thing when I stripped down my PFK Cube a while ago. In a heavily planted aquarium it is pretty hard, if not impossible, to disturb the substrate regularly - are such patches something to be concerned about long term or are they a natural thing that just happens? Can't say I have had any plant or fish related problems in either tank...the 15-gallon had been set up for ~18 months,the PFK cube for ~5 months.