Post mortem on Deep Sand Bed

[joncd]

For a 20g like the OPs configuration would Miracle Mud (or a similar such product) be appropriate where a DSB may not?

 

[3D-Reef]

For a 20g like the OPs configuration would Miracle Mud (or a similar such product) be appropriate where a DSB may not?

The OP's original goal was to grow seagrass.The problem is in setting up the sediment so that DOC's are low enough that autotrophic denitrification bacteria dominate over that of hetrotrophic denitrifiers.
Seagrasses can and do grow in muds,silts,and sand,but,in araggonite only sediments,the grass is P limited.
In mud only sediments(or >25%) they become N limited,as the bacteria out compete the grass for ammonium.
So a mixture of both,mud and araggonite,is needed for lush growth.

 

[goodreef]

I failed to put in my original post that 50% of the substrate was taken from an existing setup that was over a year old, and it was allowed to cycle for 4 months prior to planting the shoal grass.

Here's your problem!! I know cause it happened to me too.

Your "old" substrate is full of detritus and nutrients. It should ONLY be used to top up the top 1 inch level.

Imagine. In your "old" setup, the oxygen level is properly distributed from the top to the bottom. Once you move it, all of the sand is exposed to high oxygen levels. Also, however much you clean the "old" sand, it never seem to be clean.

So, basically, if you use 50%, even just for the top, your old sand is at least 2 inches deep. At this level, before your sandbed is fully established, the detritus in the "old" sand starts to decay. The decay will quicky use up oxygen ... Thus, over few weeks, all the oxygen gets depleted, and Hydrogen Sulphite (black stuff) forms.

To avoid this in the future, do not add more than 1 inch of "old" sand to your new tank.

HTH

 

[bdbyace28]

That was definately a lot of sand for such a small tank. That is very possible that the depth created a lack of water flow and subsequent lack of oxygen. Good luck with your next tank