I am not very clear on how much of actual MTS you have left since you say 38lb of soil and 20lb of pebbles, twigs and others. Does that mean you have 18lb of MTS?
Anyways, the amount left depends on the top soil you bought. If the topsoil you bought have a high organic matter content to begin with, then there is no surprise that you're getting that little of MTS as the end product. But I got about 30-40% of the total volume I started out with.
I'm no expert, but seems like 38 lbs should be enough for 3/4"to 1"depth in a 48x18 footprint tank.
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I don't think I would want any thicker layer of MTS than that. I would prefer to top that with 1.5-2 inches of Flourite, pool filter sand, or any other relatively inert substrate. I found that the MTS tends to settle to the bottom what ever you do, so mixing it with sand, etc. doesn't do much. Might as well leave it at the bottom to start, and make up the rest of the substrate with inert materirals, preferably high CEC materials.
I just finished sorting through a 40 pound bag and ended up with about 20 pounds of 'dirt' and the rest was rocks and twigs. I think it depends on the top soil and where it is collected. But for the most part, it seems like it is normal to loose a lot to debris.
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