Best for what purpose ? You'll never keep black or white sand clean.
Mineralized top soil is best for plants. And no doubt it's in the substrate section, but someone may have a link to the formula made up by some who know what he is doing.
Complete step by step on making mineralized top soil.
But then you can just use the MGOCPM that everyone uses..
I don't like knowing that my sub will expire somewhere down the line. So I prefer
an inert sub like Fluorite. I also have the Eco in one tank. Plants take in just as much from their leaves as the roots so no plant "needs" any particular sub except that DHG
does give much better growth in a dirt sub. I'm going to speculate that the leaf(if that
is what they call it)type doesn't absorb nutrients as well as regular leaves do.
But I said they don't "need" it...but they all can benefit from it so it's a choice for the owner of the tank. Based on whatever they focus on as important to them.
All subs have both good and bad points so which is more important to you ?
Cheap would be Black Diamond blasting media sold at Tractor Supply.
I use this under the Fluorite to cut the cost of the Fluorite.
Sold at some mom & pop type pet shops or you can go directly to a sand blasting yard.
Another cheap alternative and darker than white. Could be a cap for a dirt sub also.