Things i want to focus more on:
Soil and the understanding of how it directly impacts the ammonia/nitrate cycle ~ this is still relatively new to me, however my other 4 tanks now have soil (amazonia APA) and are fine.
The tank i have when all my problems began, is it possible that something could directly effect the cichlid/cory family as those are the only species which have died in this tank.
The original post has coral sand as the substrate, I am assuming no soil was involved, so I doubt soil is the issue.
As for the impact of soil on the ammonia/nitrate cycle, like most things it depends (type of soil, how much organic matter, depth, contaminants, etc)
I have been using soil in aquariums for 20 years without problems.
The biochemistry and biology of submersed soils is extremely complex, but it is not really necessary to understand it.
You have to take care with freshly submerged soils as they will release ammonia into the water (the more organic matter the more ammonia they tend to release), but his usually stops after a week or two.
If anything soil improves the biological filtration, by providing a whole lot of surface area and a whole raft of different bacteria that operate at different degrees of oxygen saturation.
Cheers
Brett