I agree with Dan that ripping things out is too drastic a measure unless you really wanted to change things up for the sake of changing. As Dan says, this ought to be yet another data point that heating cables are a construct of The Matrix and are totally illusionary.
Instead of ripping everything to shreds though why not use the tank to test some other theories? If we think about the reasons for anaerobic activity we should recognize that the direct cause is insufficient oxygen distribution within the sediment. In my opinion the depth of the substrate should only be consider a secondary factor in anaerobic activity. So how do we oxygenate the soil such that O2 is available for nitrification? Well, how about the addition of plants having extensive root systems? Assuming that they comply with your aesthetic requirements how about adding crypts and swords? You shouldn't need a lot of them, just a few good specimens placed at strategic locations.
The next component would be the addition of more CO2. More CO2 means more oxygen production which means better O2 transfer of O2 to the roots and therefore better distribution of O2 within the sediment. This is why I feel substrate depth can exacerbate anaerobic activity but is not necessarily a direct cause. As long as there are healthy roots distributed throughout the sediment, even a deep sediment, and as long as photosynthesis rate is at peak value then O2 will find its way to all sediment levels which have roots. So healthy plants create a healthy sediment. I wouldn't necessarily "blame" Duponit or cables since the Duponit probably provides some organic carbon, so that's OK and the cables heat the tank in the Winter so that's fine too. I just wouldn't give either too much credit for anything. 😀
Cheers,
